This is the same situation we all faced during the so-called "Assult Weapons Ban"
It was illegal to sell NEW "high capacity" magazines (standard capacity magazines manufactured after the ban took effect), but it was legal to possess, own, buy, or sell "Pre-Ban Magazines". You could still own, use, buy, or sell a 100 round drum as long as it was made prior to the ban.
Very few magazines were marked in any way to determine when they were made. There were magazines marked "LE Only", but even these were legal if they were manufactured before the ban (which there was no way to prove or disprove since no date was stamped on it).
As always, it was a poorly thought out, unenforceable law. The only effect it had was to dramatically increase the cost of "pre-ban" magazines (which you had no idea when you were buying them, if they were truely legal or not). Again, it was perfectly legal to buy them and no way to determine when they were made (at least no practical way, I am sure the manufacturers might be able to determine this from any serial numbers or something, but no way for law enforcement to easily determine it's date of manufacture).
FWIW: it was the same with the rifles. There were a number of evil features that were permissible. If you had more than that number of evil features (detachable magazine, bayonet lug, flash suppressor..........) how could it be determined if you owned this rifle prior to the ban, or if these features were in place prior to the ban ? You couldn't. I did hear of a guy getting busted for selling a LE Only marked AR15 at a gun show, but that is the ONE case I ever heard of where someone was charged with making an illegal transaction during the ban.
You know the rest. In the books you have read
How the British Regulars fired and fled,
How the farmers gave them ball for ball,
From behind each fence and farmyard wall,
Chasing the redcoats down the lane,
Then crossing the fields to emerge again
Under the trees at the turn of the road,
And only pausing to fire and load.