Even the ten round magazines really only applied to new production magazines since all of the standard capacity magazines were still perfectly legal to buy, sell, or own. Although at the astronomical prices you mentioned.
USGI 30 rounders were going for around $25 each at the time, that's the equivalent of about $37.50 today.
I remember a friend buying a beta drum at the time and paying $400 for it.
RATATATATATATATATATATABLAM
If there's nothing wrong with having to show an ID to buy a gun, there's nothing wrong with having to show an ID to vote.