Ess & anonymous:
Yes, that is what the "C" refers to. Many of the early 7's were installed in larger floor standing and/or wall cabinets (the case was an option).
As far as sound quality goes, I will add that a local friend had the volume pot of his 7C upgraded (think that Hovland, the tube preamp/amp guy, did the work) and his is one of the nicer sounding preamps I have ever heard.
He said that this was the only mod, but when I was checking out the work I also noticed a couple of small cast metal rectangular boxes, near the power supply that did not seem stock to me (but maybe they are).
This made me wonder what my old Pilot 690A (preamp/tuner) would sound like with a nice/budget stepped volume pot, or perhaps one of my vintage integrated amps?
The custom hand built stepped pot in the 7C was expensive, but screening/matching the resistors for each stereo step alone would be a lot of work (plus putting it together and retesting/replacing anything that ended up being out of wack).
One obstacle with some vintage tube units (in regard to volume pot replacement) is that they have a combo on/off-volume pot. Not certain if simply separating the on/off switch will work.