None of those guns are registered and even if a law enforcement agency tracked the serial number through the system back to the dealer or individual that sold them to me they would then have to prove I did not transfer the firearm prior to the date a UBC law was enacted.
That's my concern. I would need to carry around some sort of receipt or paperwork to prove I'd transferred the gun legally, turning ownership into something that requires an affirmative defense.
Let's say I'm approached by someone about a gun I have in my possession.
"Where'd you get that?"
"This? Man...I bought it from a friend of mine back around...1991 or so."
"So you've had it for awhile?"
"Yep. Got it in college. Shoots like a..."
"But you have no proof of that?"
"No, why would I?"
"Well, without proof of lawful transfer, it's illegal."
"I'll need you to turn around and put your hands behind your back, sir."
The burden of proving compliance now falls to the defendant
Furthermore, what sort of "proof" will be considered acceptable? A handwritten bill of sale? Something on the letterhead of a gun shop (that might not even be in business)? A standardized government form?