Well, for anyone curious, THAT was a 7/32" rivet. I'll say "oops" because I'd already prepped the rest of the window and removed the plastic sheeting from the window hole before drilling it out. I'm kind of regretting removing them; it was to use the already installed and unrusted parts in our existing frame.
Due to a camping trip in less than two weeks, I decided to move forward and bolt it together, with an eye towards replacing it with the proper sized rivet later (it is accessible installed).
I did the installation in the existing frame from the outside; later this summer we will pop it out and rerivet/seal it, but it's doing great at the moment.
The last picture is how it looked BEFORE we removed the Air Conditioner.
. nice change....already so much cooler!!