I have never seen it on anything but the receiver of a Colt. It was only applied after the pistol had passed all requirements and inspections.

From Colt's "A Century of Achievement".

"A Colt has to earn it V.P.,--the "verified proof" mark. It is truly a badge of perfection which the final inspectors strike on after they have given an arm a clean bill and a license to go out into the world and uphold the Colt reputation for building the world's finest hand guns."