The Expander 1&2 switch, that's the part that I'm not sure of. Also on the back there is some sort of recessed sensitivity dial ? marked "Cntl Rej".
The 1 and 2 refer to different ratios of downward expansion. The GATE setting is 1:20, I think "1" is 1:2, and "2" is 2:3, so it's basically hard gating, more gentle expansion, and even gentler expansion as you move the switch downwards. At least that's what I remember. It's been a good 5 years since I owned one, although I had several for the 20 years before that. Some experimentation with a tone should help you figure it out.
The CTRL REJ is very similar to what is outlined in the KEPEX manual that Art linked to. If you feed a signal into the KEY input while listening to the output, you can adjust that pot for minimum leakage through to the output. I think they recommended using a kick drum signal. Not sure why that would be better than a tone. I never did bother to tweak any of the units I owned.
A couple other things I remember: The inputs are balanced, but the outputs are only single-ended. The KEY input is normalled to the regular INPUT, so plugging something in that jack automatically puts you in external key mode. The signal path is very simple. It basically goes from the input balancing circuit, to the VCA, to the output driver. All the other stuff is just sidechain control of the VCA. These were originally made by a company called US Audio (that name was still screened on the internal PC board. Now it seems to be a Whirlwind brand.) in the early '80s, but later picked up by Valley, who changed the packaging slightly.
There's a slim chance I still have a manual somewhere (can you believe I used to read those things?). If I run across it, I'll check my facts, and perhaps make a scan of it.