Another tip shop find – this time a PACOM 3170 video “thing”.
I realised after I made the video that the identification overlay that I think this produces would require a video source to generate that, and the video cards don’t have that, and it’s unlikely the microprocessor is bit-banging the video. BUT, I missed the obvious – there’s provision for a daughtercard on each video card, with pins labelled “VSYNC, HSYNC, ID VIDEO” amongst others on one side, and the other side has serial in/out, ID0-3 pins (device select?).
So this one wasn’t generating overlay video because it doesn’t have the daughtercards.
Given that stamping the time on the image would require much the same, it’s possible the daughterboard generates the time also.