Here is some info from the net regarding the "Standard Trap" marking. However, the op's gun is also stamped for Super Speed & Super X 3":
In the mid 30's until around 1942 WW offered the Model 12 in 12, 16 and 20 gauge in the Trap grade and it was stamped "STANDARD TRAP" on the receiver extension. Around 1938 WW started offering a second Trap grade Model 12 in 12, 16 and 20 gauge and it was stamped "MODEL-12-TRAP" on the receiver extension. Around 1942 WW dropped the "STANDARD TRAP" stamped Trap grade guns and continued stamping "MODEL-12-TRAP" on the receiver extension of Trap grade guns until 1950 when the stamping practice of Trap and Skeet grade guns was terminated by WW. I have not witnessed a post 1942 Winchester Model 12 Trap grade gun stamped "STANDARD TRAP" on the receiver extension.
All Trap grade guns from the mid 30's until 1950 wore solid red Winchester recoil pads, checkered Trap grade stocks and forearms and most wore pistol grip caps. Some 1938-1942 Model 12 Trap grade guns, 12, 16 and 20 gauge are seen without grip caps as this is a documented option by WW. All Trap grade guns wore a solid rib barrels or a 2 pin vent rib barrel in 12 gauge only. Factory vent rib barrels on 16 and 20 gauge model 12's were not available until after 1955.
Prior to the mid 1930's Trap grade Model 12's were stamped "TRAP" underneath the serial number on the receiver, not on the receiver extension. I have never witnessed a 16 or 20 gauge gun with this stamp. I believe all pre mid 1930's Trap grade guns stamped "TRAP" under the serial number on the receiver were 12 gauge guns and 16 and 20 gauge guns were not marked by grade.