This version is presented here courtesy of YO3GHM, who owns such a specimen built in the '90s, and who recently analyzed it and extracted its schematic. Since I never personally built it, I consider it for now "In drafting stage". Below you can see a photo of the circuit, as it was sent to me by YO3GHM.
Next, the schematic extracted, drawn and sent to me by YO3GHM.
Schematic - v.2
The binary image of the PROM (extracted by YO3GHM) along with the schematic below, can be downloaded as a ZIP archive.
/DSC00002.JPG)