I'm thinking its a K-22 3rd model and he looked inside the crane for the SN since many S&Ws and other revolvers have the SN there.

If he took the SN of the butt, I suppose it could be a pre model 34, 35 or 43. Anything else is a six digit or has a prefix.

Many people don't know to check the butt for the SN because they always check the crane or because the grips cover the butt of the frame.