ATMachine said:
Times change. The SE of THX 1138 was rated R in 2004, though the previous versions had been rated PG back in the 70s (and that was back when there was no PG-13 rating!).
The original release actually carried the now defunct GP rating...
http://rating-system.wikia.com/wiki/GP_rating
The 1978 post Star Wars reissue was PG of course.