[Guitar] Other than reverb, what was the difference between the "Princeton" and "Princeton Reverb?"

It's just the reverb and an additional gain stage. Fender made these 2 amps because of the difference in cost, it allowed them to offer a lower cost amp.

Tonally the difference between the circuits is that the Princeton has less gain/overdrive than the Princeton reverb. Fender added a gain stage to recover the lost gain in the reverb section. It's a bit of an overcompensation so the Princeton reverb is easier to drive into that sweet grind these amps do so well.

FYI, the Princeton was my first real amp, Princeton reverb is my current go to amp for guitar for small gigs. I built it several years ago with only 2 small mods, I added a bright switch and a mid control.