Depends on how you obtained it.
If you bought it from a gunshop...there is paper, NICS, etc. Regardless of what "they" say, that is de facto registration...hence they know you had it on that particular date. If you bought it privately, there is no paper directly linking it to you...but there is on the past owner.
Granted, they can make it difficult for you, but private sales are still legal without "paper"...thus if they did track you down, you could say you sold it...at a gunshow, or to a friend of a friend, etc. They would have to prove that you were hiding it in order to charge you. So, at this point in time, if you indeed did sell or give it away, you have comitted no crime...thus you couldn't be prosecuted for not registering a firearm you don't have, and they can't prosecute you retroactively. Now, thats the letter of the law...what the courts do these days is a crapshoot. So, anything you do is a matter of conscience.
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"Quis custodiet ipsos custodes" RKBA!