But in this case the museum doesn't have a better claim than you do either that they aren't the thief, just an older one. In fact theirs seems worse, just a record that they possessed it at one point in the 1920s. Could be that the curators who first logged it into inventory were the original thieves in the line plundering it from someone else vs paying fairly.
In the legal perspective, none of that matters. If the artifact can be definitively proven to have belonged to X, and they were robbed, then it belongs to X regardless of who's done anything in the meantime. That's the legal answer here. The same is true of real property, or a stereo. It belongs to the person it originally belonged to, before it was stolen.
Morally, it's debatable. Legally, it's straightforward, a stolen item does not become un-stolen with time, nor does the claim of the original owner diminish. Morally that's not how people think, but legally that's how it works.
The British Museum argument does not work for plebians. It's not "yours" just because you kept it for 100 years. Nor is it the next person's because they didn't know it was stolen.
The British Museum only works for the British because they were a god-ordained royal family that controlled an empire spanning the globe, then a nuclear-armed imperial power. It was based on the rather debatable legal theory of "try and stop us". It's increasingly recognized that it's unjust and seems increasingly likely that it will be dissolved sometime in the next hundred years. Because yeah, the same arguments apply to shit the British looted as shit the Nazis looted. It's not theirs. Maybe some countries will be willing to loan them back on a permanent basis though.
It seems to me if they want to have a legitimate claim strong enough to take it from someone who legitimately purchased it, then they will need to come up with some evidence that they themselves did not steal the statue from somewhere else in the first place.
