/usr/share/initramfs-tools/scripts/casper-bottom/19keyboard is in casper 1.340.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
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PREREQ=""
DESCRIPTION="Setting up console keyboard..."
prereqs()
{
       echo "$PREREQ"
}
case $1 in
# get pre-requisites
prereqs)
       prereqs
       exit 0
       ;;
esac
. /scripts/casper-functions
log_begin_msg "$DESCRIPTION"
kbd=us
cslayout=
csvariant=
csmodel=
for x in $(cat /proc/cmdline); do
        case $x in
                kbd-chooser/method=*)
                        kbd=${x#kbd-chooser/method=}
                        ;;
                console-setup/layoutcode=*)
                        cslayout=${x#console-setup/layoutcode=}
                        ;;
                console-setup/layoutcode\?=*)
                        cslayout=${x#console-setup/layoutcode\?=}
                        ;;
                console-setup/variantcode=*)
                        csvariant=${x#console-setup/variantcode=}
                        ;;
                console-setup/variantcode\?=*)
                        csvariant=${x#console-setup/variantcode\?=}
                        ;;
                console-setup/modelcode=*)
                        csmodel=${x#console-setup/modelcode=}
                        ;;
                console-setup/modelcode\?=*)
                        csmodel=${x#console-setup/modelcode\?=}
                        ;;
                keyboard-configuration/layoutcode=*)
                        cslayout=${x#keyboard-configuration/layoutcode=}
                        ;;
                keyboard-configuration/layoutcode\?=*)
                        cslayout=${x#keyboard-configuration/layoutcode\?=}
                        ;;
                keyboard-configuration/variantcode=*)
                        csvariant=${x#keyboard-configuration/variantcode=}
                        ;;
                keyboard-configuration/variantcode\?=*)
                        csvariant=${x#keyboard-configuration/variantcode\?=}
                        ;;
                keyboard-configuration/modelcode=*)
                        csmodel=${x#keyboard-configuration/modelcode=}
                        ;;
                keyboard-configuration/modelcode\?=*)
                        csmodel=${x#keyboard-configuration/modelcode\?=}
                        ;;
        esac
done
# TODO: This is a horrible clone-and-hack from console-setup. We should
# definitely be calling console-setup instead; however, that's too risky for
# 8.04 since it will need careful installer testing and will probably be
# slower.
adjust_console_setup () {
        if [ -z "$csmodel" ]; then
                if [ "$cslayout" = br ]; then
                        csmodel=abnt2
                elif [ "$cslayout" = jp ]; then
                        csmodel=jp106
                fi
        fi
        latin=
        case $cslayout in
                jp)
                        case $csvariant in
                                ''|106|common|OADG109A|nicola_f_bs)
                                        latin=yes
                                        ;;
                                *)
                                        latin=no
                                        cslayout=jp,jp
                                        csvariant="106,$csvariant"
                                        ;;
                        esac
                        ;;
                lt)
                        latin=no
                        cslayout=lt,lt
                        case $csvariant in
                                us)
                                        csvariant=us,
                                        ;;
                                *)
                                        csvariant="$csvariant,us"
                                        ;;
                        esac
                        ;;
                me)
                        case $csvariant in
                                ''|basic|latin*)
                                        latin=yes
                                        ;;
                                cyrillicyz)
                                        latin=no
                                        cslayout=me,me
                                        csvariant="latinyz,$csvariant"
                                        ;;
                                cyrillicalternatequotes)
                                        latin=no
                                        cslayout=me,me
                                        csvariant="latinalternatequotes,$csvariant"
                                        ;;
                                *)
                                        latin=no
                                        cslayout=me,me
                                        csvariant="basic,$csvariant"
                                        ;;
                        esac
                        ;;
                rs)
                        case $csvariant in
                                ''|basic|latin*)
                                        latin=yes
                                        ;;
                                yz)
                                        latin=no
                                        cslayout=rs,rs
                                        csvariant="latinyz,$csvariant"
                                        ;;
                                alternatequotes)
                                        latin=no
                                        cslayout=rs,rs
                                        csvariant="latinalternatequotes,$csvariant"
                                        ;;
                                *)
                                        latin=no
                                        cslayout=rs,rs
                                        csvariant="latin,$csvariant"
                                        ;;
                        esac
                        ;;
                am|ara|ben|bd|bg|bt|by|deva|ge|gh|gr|guj|guru|il|in|ir|iku|kan|kh|la|lao|lk|mk|mm|mn|mv|mal|ori|pk|ru|scc|sy|syr|tel|th|tj|tam|ua|uz)
                        latin=no
                        cslayout="us,$cslayout"
                        csvariant=",$csvariant"
                        ;;
                *)
                        latin=yes
        esac
        if [ "$latin" = no ]; then
                csoptions="${csoptions:+$csoptions,}grp:alt_shift_toggle"
        fi
        if [ "$cslayout" != us ] || [ "$csvariant" ]; then
                csoptions="${csoptions:+$csoptions,}lv3:ralt_switch"
        fi
}
PREFIX=
CONFIGFILE=
if [ -f /root/etc/default/keyboard ]; then
        PREFIX=keyboard-configuration
        CONFIGFILE=/etc/default/keyboard
elif [ -f /root/etc/default/console-setup ]; then
        PREFIX=console-setup
        CONFIGFILE=/etc/default/console-setup
fi
# Make a temporary copy of the current file for later diff
cp /root$CONFIGFILE /tmp/keyboard.orig
# Reset the config for persistent medias
if [ -f /cow$CONFIGFILE ]; then
    echo "Resetting $CONFIGFILE"
    cp /root/rofs$CONFIGFILE /root$CONFIGFILE
fi
if [ -x /root/bin/setupcon ] && [ "$PREFIX" ] && [ "$CONFIGFILE" ]; then
        if [ "$cslayout" ]; then
                csoptions=
                adjust_console_setup
                chroot /root sed -i "s/^XKBLAYOUT=.*/XKBLAYOUT=\"$cslayout\"/" \
                        "$CONFIGFILE"
                if [ "$csvariant" ]; then
                        chroot /root sed -i "s/^XKBVARIANT=.*/XKBVARIANT=\"$csvariant\"/" \
                                "$CONFIGFILE"
                else
                        casper-preseed /root "$PREFIX/variantcode" '' false
                fi
                if [ "$csmodel" ]; then
                        chroot /root sed -i "s/^XKBMODEL=.*/XKBMODEL=\"$csmodel\"/" \
                                "$CONFIGFILE"
                else
                        casper-preseed /root "$PREFIX/modelcode" '' false
                fi
                if [ "$csoptions" ]; then
                        chroot /root sed -i "s/^XKBOPTIONS=.*/XKBOPTIONS=\"$csoptions\"/" \
                                "$CONFIGFILE"
                fi
        else
                casper-preseed /root "$PREFIX/layoutcode" '' false
                casper-preseed /root "$PREFIX/variantcode" '' false
                casper-preseed /root "$PREFIX/modelcode" '' false
        fi
        casper-preseed /root "$PREFIX/optionscode" '' false
        # prefix remains console-setup even with keyboard-configuration
        casper-preseed /root console-setup/codesetcode '' false
else
        chroot /root /usr/sbin/install-keymap $kbd
        casper-preseed /root debian-installer/keymap "$kbd"
fi
if [ "$(cat /root$CONFIGFILE)" != "$(cat /tmp/keyboard.orig)" ]; then
    # The keyboard settings have changed, gsettings needs to be flushed
    if [ -f /root/home/$USERNAME/.config/dconf/user ]; then
        chroot /root sudo -u $USERNAME dbus-launch --exit-with-session gsettings reset-recursively org.gnome.libgnomekbd.keyboard
        mount -n -o bind /proc /root/proc
        chroot /root killall dbus-launch
        umount /root/proc
    fi
fi
log_end_msg
 |