I have an opinion that differs just a bit. First of all, there was a difference in the diameter of the pins that coincided with different timeframes. Prior to the end of WWII, the pins were a larger diameter than those post war. Here are some examples:






There were (Of course) some exceptions. Most notably were the carvers. I have not seen any with the larger diameter pin in any timeframe. Another data point is John'a prewar Hunter:




Post war there weren't many two pins at all, and none with the larger diameter pin. Those few that have turned up have the small stamp if they were Model 3's or Model 4's. They are also typically paired with a Heiser sheath with oversized throat rivets. Here's one example and Joe's 3-6 as depicted is another. Bottom line is that I think two pins went away just a few months after Bo brought out his new line of knives in the summer of 1945.