I have multiple copies of it. It is in 4.3BSD and 4.3BSD-Tahoe. I also purchased the CD sets from McKusick. The oldest copy I have is dated June 5, 1986 from 4.3's /usr/contrib/tac/
I put the README, source, and SCCS files here: https://ftp.netbsd.org/pub/NetBSD/misc/reed/tac/local/ I export the sccs (like history and blame) for the tac.c here: https://ftp.netbsd.org/pub/NetBSD/misc/reed/tac/tac.c.sccs.history.txt
