K721070 is from 1967. It is right near the end of 15-2 production. The dash 3 was introduced that year, with the difference being that the sight leaf screw was moved away from the barrel/cylinder gap.
The .38 Combat Masterpiece is among the best of the postwar hand ejector adjustable sight revolvers. Many of us carried them daily in Viet Nam. For what you described, $550 is very decent price. I would be down there purchasing it now if I were in your shoes.
Here is a dash 2 I received as a gift from my late father* many years ago:
* We lost Dad eight years ago today. A veteran of the D-Day invasion, his P-38 was shot down by FlaK ten days later. He survived 9 months as a POW of the Germans and was liberated by Patton's 3d Army in 1945.