/usr/share/ltsp/xinitrc.d/I05-set-ltspfs_token is in ltspfsd 1.4-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | #
# sourced with .
#
# Set authentication token for ltspfs in the session
#
if boolean_is_true "$LOCALDEV" ; then
    ltspfs_token=/var/run/ltspfs_token
    if [ ! -f "$ltspfs_token" ]; then
        # ltspfsd may not have been started yet if no devices are plugged in,
        # so create the token now.
        mcookie > "$ltspfs_token"
    fi
    xprop -root -f LTSPFS_TOKEN 8s -set LTSPFS_TOKEN $(cat $ltspfs_token)
fi
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