I have a pdf of a 1938 graflex catalog that clearly describes a Series B 5x7 "with stationary back" to distinguish it from the 2 1/4 x 3/14, 3 14 x 4 1/4, and 4x5 RB models. (a good description here:
https://lommen9.home.xs4all.nl/graflex/Series B.html ). I assure you mine is not a revolving back. If it was I would be keeping it for wetplate work. Its serial number, 163352, puts it right around a batch of Series B's of unspecified size around 1928.
I agree it is not in the same league as the Home Portrait, which has limited movements, longer bellows, interchangeable lenses and a rotating back, but in terms of scarcity and use I think it is far superior, at least for my work in portraits, to a speed graphic. It has longer bellows and can be shot handheld with instant ground glass focusing via the RF on the speed. I know when it comes to 4x5 I use my Series D far more often than my Speed. To my understanding the only difference between it and the Press Graflex, other than seeming to have had far fewer produced, is the Press Graflex's 1/1500 shutter speed. The Series B actually essentially replaced the Press Graflex, at least in 5x7, starting production in 1923 when the press graflex ceased production.
Still, I think 200-400 seems low, but maybe thats just me. If I were an expert I wouldn't be here asking everyone else.