Tom, thanks for pointing out something that, now that I take a closer look, is pretty obvious. I think that if I were Irish, my name would be O'Blivious, but anyway, I started the correction this morning. First step, removal of my added weld, down to the lip that's on the original MP turret front.

It's a bit rough, but a little cleanup later,

It's pretty close now. I may finish the cleanup (I had to use a little MrSurfacer) and move on. There's a lot of little parts on the turret there and I'm trying not to go too far backwards. From looking at The Book, I'm thinking the mantlet extension bolted in from the inside, in the original opening, with the flange added to keep squirrels and birds out....
I have a Dragon M50 turret that misses this detail (the area looks to have a very this weld bead, and an old Verlinden M50 turret that looks like a better attempt but still no banana. The Verlinden bustle extension weld and cut line is also incorrect. Hmmm.
Edit: I also have to replace the big slotted screw-heads on the top of the mantlet extension. Workin' on that.....