/usr/share/acl2-4.3/books/str/fast-cat.acl2 is in acl2-books-source 4.3-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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(ld "cert.acl2") (certify-book "fast-cat" ? t :ttags :all)