I suspect they went back into the garage and found a lump hammer.
I have tested with a fairly good meter continuity between the socket connectors and the solder tags on the PCB. AOK.
I have with heart in mouth squirted WD40 into the socket connectors and rubbed fine pins in and out of the connectors to de-tarnish the connectors.
No change.
But when I wiggle the lead to get the output back, with the side off the machine, I can see I am flexing the PCB significantly. I suspect a broken track or a dry joint elsewhere on the graphcs card PCB. At this point I give up. I am not going to try manually to re-solder every solder joint on a graphics card, nor to try to trace and solder fly-leads across tracks on a graphics card PCB.
I suppose I could have a look at the BIOS settings and see if I can find anything that might get the Matrox card up on the PCI bus. Or I can put the data off the C-drive onto the D drive, put the D-Drive onto the spare machine, and see if I can get the spare machine to work as a print driver, while I save up for a new graphics card.