What do you mean by 2 pin or 3 pin?
A 2-pin rib is a vented rib that came out early in production and ended around 1954. It's an attractive and sought after factory vented rib but became very costly to produce because the rib itself fastened to and over posts that were milled into the barrel. Now these posts were turned down on Winchester's offset lathe and required a lot of handwork, by skilled tradesmen, to produce the rib posts. Then the rib itself was milled and machined, from a solid piece of Winchester steel, producing the rib and the outside posts that slid over the barrel posts. Where the magazine tube ends the rib was secured to the barrel posts with "Two-Pins" giving it the 2-pin rib name.
Around 1954 the cost was too extreme to be competitive with Remington's 870, that used stamped parts, WWII technology and bolt lock-up in the barrel extension. So Winchester made a deal with Simmons, who had perfected the doughnut post rib, that used rib post brazed to the barrel with just rib itself sliding over the posts. This was first done at the Winchester factory with Simmon's supervision. This shaved a considerable amount off the retail price with the 2-pin rib still available for a couple years after but at a steeper price.
A couple years later they trimmed the cost again using Simmon's round post rib. Now these Simmon's ribs that were installed at the Winchester factory are the real Winchester deal and considered correct but the same ribs installed at the Simmon's factory are so marked on the side of the rib and are not considered to be original Winchester and M12 experts adjust the current value considerably.
By the Late 1950's Winchester still under the gun for production cost increases compared to retail selling price came out with the "New Winchester Vented Rib" referred to as the "Three-Pin Rib". The rib posts, instead being milled as part of the barrel, were brazed on to the barrel to hold the rib that was secured using 3-pins. The 3-pin rib M12's are desirable and hold a considerable premium over a plain barrel but do not command near the price as the original "Two-Pin M12's.
When I have some time I'll take some pictures of some of my M12's. I've being using M12's since I was a boy and when I got older began buying more of them but the last 10 or so years I've culled them down to vented guns, smaller gauges in high condition as a priority. One I found, early this year, is a 1948 20 gauge, solid rib, Skeet marked gun with WS-1 choke with beautiful figured wood. The gun is still on the original take-up notch with absolutely no bolt droop and in excellent original condition with original buttplate. Now these are the ones you want to hold out for. They're expensive but they don't make them anymore.
