I was just taking my grips off my 1961 Official Police and saw on the right side of the rear end of the frame the letters INS under where right grip goes. Never noticed that until now. What does it mean?
INS is a factory inspection stamp often found on Colt mid-frame revolvers made in the 1960's.
It simply signifies that the frame was inspected and passed.
You'll find other inspector's stamps on the left rear side of the trigger guard, the Colt Verified Proof stamp, which looks like a "VP" in a triangle on the front left side of the trigger guard, various stamps on the grip frame, and often letter stamps just below the barrel on the frame around the serial number.
All these are applied at various stages of manufacture to signify that the gun was inspected and all work done up to that point was acceptable.