Riverdog,
If it is a pre-model number K-22 (pinned, recessed, either four or five screw) then the only way to determine the model is the barrel length and front sight. A Masterpiece (pre M17) will normally be six inch barrel and have a Patridge style sight. A Combat Masterpiece (pre M18) will have a four inch barrel and a ramp style sight. Of course if it is one of these, the number under the crane isn't the serial number (it's an assembly number from the factory). The serial number will be on the bottom of the grip strap, the back of the cylinder, and possibly on the bottom of the barrel above the ejector rod. A four screw model has three screws in the sideplate (the rear lower screw may be under the grips) and a screw in the front of the triggerguard. A five screw has one more in the sideplate at the top (just below the rear sight).