It turns out that the gun was a rare and expensive German StG 44 from World War Two.
WW2 weapon turned in during gun buy-back program in CT
Fortunately, the nice police officers realized what it was and are arranging for her to be able to sell it for what it's worth instead of giving her a $50 gift card and sending it to the smelter with the rest of the junk. Good on them for being so decent about this.
Sadly though, unless this weapon was registered during the brief 1968 Amnesty for such war trophies, it's likely to be declared contraband by the feds and thus cannot be sold for anything except parts. I really hope that's not the case here though. I'd like to see that old weapon wind up in the hands of a proper collector or museum. And there are still a lot more old war trophies like that one out there, sitting in attics or at the bottom of trunks, long forgotten. This is why we need a change to the NFA laws allowing these historic items to be properly registered and thus made transferable. (Yes, and the whole NFA ought to be abolished, but that's not going to happen with the current administration...not in a million years.)
Excuse me now, while I go shed a tear or two for that beautiful old soon-to-be-destroyed rifle.