"Semi-automatic weapons and magazines have been around for years (decades?) prior to 1973."

Yep.

Even the evil, scary AR-15 has been available for nearly 53 years. Page 19 of the November 1964 issue of Guns magazine had a full-page advertisement from Colt for the "AR-15 Sporter". A semi-automatic rifle chambered in .223 Remington, with a 20-round magazine.

Available from any Colt dealer for $189.50 (about $1500 today, according to https://data.bls.gov/cgi-bin/cpicalc.pl). Back then, of course, it could be delivered to your door with no background check.

http://gunsmagazine.com/1964issues/G1164.pdf