Originally Posted by legalsten
Mine is the 3 pin design and I was guessing that somewhere down the line they simplifed by using one frame for each given size ( full , compact , sub compact ) instead of one for the 9 mm and another one for the 357 and 40 .
Thanks for the info
Checking a few Glock full-size 9mm handguns/photos that I can access, I see that by the DT* ***-series of serial numbers, there was a faint shadow mold-mark on the frame where the third pin hole would eventually be, like they were already using the 3-pin .40 frame mold, but plugging the hole for the third pin when they used it for 9mm frames. I saw the same thing in a DU* ***-series 9mm, so they still hadn't converted to the three-pin frame for the 9mm guns at the time that Glock was made. Checking the user database for date will give you an idea of when those guns were made, if you want to narrow-down the time window.