Here is a picture of VH barrel flats stamped with the
TESTED PB
U.S.A.
216976 02 V in circle, TESTED PB U.S.A..jpg
From my observations, I begin seeing the OVERLOAD PROVED stamp in the 217xxx range but still see guns into the 219xxx range without the stamp.
Probably what the OVERLOAD PROVED stamp really means is that someone in marketing decided they need such a stamp to match the other manufacturers proof stamps touted in their advertising -- FOX PROOF, Winchester Proof, Remington English Proof (REP), etc.