my FT-102 has a switch on the back labeled external receiver and my 
TS-940 had one as well.
> Was the purpose of this to use the external receiver to listen, 
> while sharing a single antenna and when the controlling rig was 
> keyed, the external receiver muted?
Michael, that's basically how it worked. Although I never used the 
external receiver option, my old Swan 500CX also had that capability.

So did the Drake TR4.

> Did you have to use 2 separate antennas as well in this kind of a 
> setup?

No. You could do that if you wanted too, but you would then have to 
come up with a way to switch the second antenna to the external 
receiver, and remove that antenna during transmit. That could easily 
be done with the proper relays or electronic switch, but most people I 
knew just used one antenna.

Some rigs only provided the contacts to power an external relay that 
could be added for that purpose, much like the switching relay for an 
amplifier.

People would often use a separate receiver rather than buy an external 
VFO, especially if they already owned a decent receiver.