Like I said, I failed to save the original bios properly. I thought I had, but it was the first time I was using the CH341A programmer and afterwards I realized I'd got the wrong size of chip selected, so I only saved the first part of it, by then I'd already overwritten the bios contents with the first of the files I tried. If I restore from the saved file now the computer won't boot.
I tried various other GTS450 and GTX550 bios files I downloaded, but the only one I've tried that allows the computer to boot is the FABIOMODFAKEGTX960.rom - and that produces a mostly white screen with only little bits showing - not sufficient for getting a gpu-z screenshot.
I was hoping that someone would recognize my card from the photos and say, "I have the same one, and here is a rom file that works for it."