That's a Colt inspectors stamp.
It's probably the LETTER O, not the number Zero.
There are three main inspectors stamps on Colt revolvers.
On the left front of the trigger guard is the "VP in a triangle" which is Colt's Verified Proof stamp.
On the left rear of the trigger guard is what is assumed to be the assemblers stamp.
Under the serial number on the frame is what is assumed to be the final inspectors stamp.
It's probably the LETTER O, not the number Zero.
There are three main inspectors stamps on Colt revolvers.
On the left front of the trigger guard is the "VP in a triangle" which is Colt's Verified Proof stamp.
On the left rear of the trigger guard is what is assumed to be the assemblers stamp.
Under the serial number on the frame is what is assumed to be the final inspectors stamp.