If there is a Winchester proof stamp "WP" on the right side just forward of the receiver ring your rib installation was done for the factory even if done by Simmons. Simmons did factory contract vent rib work for Winchester. When a M-12 was destined to have a vent rib installed the proof mark was placed on the side of the barrel as not be covered by the vent rib.
It was always speculated among M-12 collectors Simmons Gun Specialty's had a Winchester M-12 proof mark die to strike a new "WP" proof mark when called for if a shotgun had a vent rib added aftermarket. This speculation also included having a "Pigeon Grade" die to strike the magazine tube making a field grade into a Pigeon Grade.
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