Evidence folks can pull old serial numbers wihout too much difficulty, and I think that the serial number requirement did not kick in until the Gun Control Act of 1968. If it was made prior to that, it may be legal. Bear in mind though, that it might be a stolen weapon, especially if it was a US property marked M1911.
I am not sure what the legal route would be to find out the serial number and legal staus of this weapon would be. You would probably be okay to work through your local PD, but they might not want to spend the time to raise the serial number and run the weapon, and you could ppossibly face criminal charges, though that is somewhat doubtful if you are going to the police to "do the right thing." Getting the weapon back is not assured, though. Working through a gunsmith might work. It might be best to just turn it in, though I would work that through an intermediary like your lawyer, for your own protection, and let the chips fall where they may.