Here it is with the ribbon cable unravelled. There is a 28-pin socket for the EPROM being programmed, which must be a 21V device. These are a bit rare nowadays, so for this reason, the Uviprom programmer became less useful to me when I ran out of old 21V EPROMs and started buying new 12V ones.
There is a space for a variable resistor at VR1, which I think can change the programming voltage down to 12V, so perhaps I should fit it one day.