First of all, mystery solved – the remote enable input for the IF attenuator. After quite painful tracing of wires and disassembly of the logic boards, a broken trace! Just a little crack, but big enough to block the electrons’ flow. A little bit of solder, and then, suddenly, the MSR-904A’s IF attenuator can be remotely controlled.
Today, a also the dual pot for the F2 adjustment arrived – fitted, also this, working again.
But most interesting, a little box, with a label not seen before:
The part has a NSN number, original unit price was no less then 71.38 USD!
Enclosed – the most special mains power cable I have ever seen: