Did the 1st gen DPMS guns use a different magazine? I don't know the answer to that question but if they did that could be the answer to your problem. Just an idea seeing the reply you got from the 1 vendor.
Gas tube looks good. Finishing nails work sometimes. I made a take down pin detent out of one once. It worked till I replaced the one that flew across the room.
I have that scope on my PA10. Yes no AO. Decent glass and the Nikon spot on app works with the retical. not bad for the money. I also have the 3x9 on my Beowulf. Works for me till I can afford something better.
Gap in the middle is where it clamps to the buffer tube. You could adjust the lop a bit by where you clamped it down tight. There are 2 nuts and bolts to tighten to clamp the brace down.
I had the trp operator with the full length rail. Nice gun. Softest shooting 1911 I ever shot. Shot USPSA and idpa with it for a couple years. Great gun wish I hadn't sold it but at the time I needed to pay some bills.