This question comes up often here, so here's what I found: Simple possession of a gun with a removed serial number is a 1st degree misdemeanor. If YOU did the removal then it's far more serious. But just having it in possession is a misdemeanor. MAXIMUM sentence is one year + $1,000 fine. (hint: maximum sentences are usually for maximum criminals, & even THEY often plead out to less)
So what would have to happen for this to become an issue for a regular guy with no criminal record or other illegal activity that attracts police attention? Because in over 40 years of shooting & collecting I have never, ever, had any authorities check the numbers of any gun of mine. That is, outside of selling or buying them.
Plus, with my conversations with the CALIFORNIA DOJ (CA is strict!) they have little concern over low-capacity antique/vintage guns owned by legal possessors. Their priority is criminal misuse, especially of illegal &/or stolen guns by crooks and prohibited possessors. Not arresting & jailing clean-record misdemeanor offenders. But they will surely confiscate any serial-altered gun that comes to their attention. Not because they care, but because it's their job.
So what to do? My legal recommendation is to immediately call the Govt & rat yourself out. Give up the gun.
BUT............... if someone was so much of a criminal that they did such acts as jaywalking or failing to declare garage sale profits, they could do this: Keep & enjoy it forever. Never transferring, pawning, it or getting it gunsmithed.
Because even if caught (how?) what would happen to this otherwise clean scofflaw? Even in Ca, probably not much.