Yes it was added on the very first Mac, and carried forward on newer compact Macs, losing signatures as the case design evolved https://www.folklore.org/StoryView.py?project=Macintosh&stor...
As for other machines, I don't know of other signatures, but there are often Easter eggs. Commodore machines had funny stuff on their PCBs: codenames Rock Lobster, Junebug, plus the Fred/Wilma LEDs, etc. Lots of integrated circuits from several companies had artwork in unused areas of the silicon die.