There is no difference in pre-64 Model 12's. They just stopped marking them trap.
The "Y" series used an investmant cast receiver and many of the small parts
were also cast. The receiver and all parts in the pre-64 were machined.
There are some variations in the pre 64 traps w/ respect to the Vent ribs. The most desirable is the 2pin aka milled rib. Next in desirability would be the 3pin rib. Least desirable would be the Win Special VR aka the donut post rib.
Many ribs have been put on model 12s aftermarket. Many different companies did it. They were glued, soldered and brazed. Most of the 2 pin guns had the top of the receivers also milled. As Skybuster said, internally, trap guns were about all the same, till the Y models. They have a Y in front of the serial number.
The entire subject is too involved to cover here.
How to tell a milled rib? On milled rib guns there will be a a longer post just above the barrel band , that is the band that holds the mag tube to the barrel. The longer post will have 2 pins through the post to hold the rib on. There is no way to fake one. Available from 1919 to 1960. You can still obtain a proofmark stamp so beware. Sorry to disagree but the Milled rib is most desireable followed by the round post then doughnut and finally the three pin because of rarity. Jeff