Based on that, I think you've got a serious problem. The 10-6 was produced from 1962 to 1976. The early models would have had a "C" prefix, which changed to a "D" prefix in 1968. With the digits of 603162, your gun presumably could be either from the very early production (1962) in which case it should have the "C" prefix, or from 1974 with a "D" prefix. But I don't see any way it could have originally had only those six digits.
At this point you need some counsel from someone who understands the legal situation and what your best options might be, and that's not me. In your shoes, I'd be inclined to think first about getting ahold of who I got the gun from and un-doing the trade, if that's possible. Whatever you do, I'd proceed with all deliberate haste.