Jay -
I don't know what "machined-in side frames" means either. Maybe ask Pete?
Anyway, I learned something new about the two different P&D bolsters. I never worked on both an F and a GP at the same time so I didn't have a chance to mix them up.
I do want to encourage you to at least download the TB 1925 and TB 2008 module data sheets from Parts Express and look at the dimensional drawings to see if either one would fit in your installs. TB modules sound so much cleaner and deeper than even 2" high-bass speakers I've installed in the past. FYI you can cut off the mounting tabs without letting the air out.
Bob S