I recently found this on ebay from an estate sale. Looks good to me, but not here yet... so I don't know about operation. Being 455kHz, it should work with an R-390 or SP-600. A friend inquired about it, and I said I'd show it to him here, and he has now sent me a copy of the manual! Thanks a million Mike!!!

I Have a very nice CV-591A/URR MAR4 SSB converter. It should  work just fine with my R-390-A, or any similar 455kHz IF receiver. I plan to get it hooked up ASAP.

The GPD-111 was made by the Government Products Division of Scientific Atlanta, as a SONAR/VLF Receiver for the US Navy. It covers 700 Hz to 100 kHz with modes: AM, LSB, USB, CW. I plan to build a relatively large loop using ribbon cable with the wires cross-connected in series, plus a separate two-turn loop (from the same ribbon cable) to couple to the receiver. See also: <GPD-111>