Thanks! I even called multiple West locations that apparently had it in stock, just have them tell me they couldn't find it.
It's a 927 part number G9271111
17 1/2" x 12 1/2" opening
17 7/8" x 12 7/8" cutout
19 7/8" x 14 7/8" flange
I'm though bolting them into a window. The hatch will just fit, but the spacing of the mounting holes is going to be the real determining factor.
Worst case, I'm using a few of them in another location too, so I can just measure them myself, but it would be nice to know ahead of time and order all of the hatches, whether all 927's or a few of them, and 2 of the next size down, all at once.