This is probably the most interesting thing I’ve found.
The switch labelled NORMAL / MINIMAG may not be an arbitrary modification. MINIMAG was a professional tape synchronizer manufactured by Automated Processes Inc. (API), designed to synchronize multiple tape machines.
I found several historical references:
- Studio Sound, January 1976
This issue includes a MINIMAG II connection diagram showing the synchronizer, an optional capstan driver and capstan control connections. This is particularly interesting given the mystery XLR connector and additional PCB on my machine. - Recording Engineer, April 1975
Contains an advertisement and technical description of API’s MINIMAG II synchronizer. - API company history
API’s own history confirms that the company developed tape synchronizer systems in the 1970s.