I don't think the Mod 91 ever shipped. It was a combat machine designed to squash CDC (IBM was sued by them for their anti-competitive actions here) and was eventually transmogrified into the Mod 95/195 which did ship.
There were approximately 15 Model 91s produced. The Computer History Museum has the console from one, and the Living Computers Museum has another.
The Model 95 was the even more rare version with thin-film memory instead of core memory. Only two of them were produced, for NASA.
The Model 195 was a reimplementation of the Model 91 with "monolithic" integrated circuits instead of hybrid SLT modules. The System 370/195 was very similar to the System 360/195. Curiously, the System 360/195 has the black styling of System/370 control panels, rather than the beige control panel of System/360.
According to wiki, there were 15 model 91s, 4 kept by IBM and the rest went to customers including NASA. There were two 95s and both went to NASA (and really the 95 was a 91 with a different RAM technology but the same CPU).
That lines up with my source who started his career doing simulation and analysis of rocket propellent in micro gravity for the tail end of the Apollo program and some of the initial work for the space shuttle.
