I'm not sure of the total history, but "by Adrian Mariano" means macOS uses GNU units. Wiki says they forked a copy before the license change.
The one in FreeBSD is from NetBSD: https://svnweb.freebsd.org/base/head/usr.bin/units/units.c?v...
The NetBSD version is from Adrian Mariano: http://cvsweb.netbsd.org/bsdweb.cgi/src/usr.bin/units/units....
The one in Plan9 is in yacc: http://9p.io/sources/plan9/sys/src/cmd/units.y
This GitHub repo claims to be V6: https://github.com/memnoth/unix-v6/blob/master/source/s2/uni...