Sorry! I should have posted pictures earlier, but I was about to head out.
Unfortunately I've not been able to find a schematic for this model, or even a similar model in the range. It is indeed Venner, as that's the name on the individual cards. Photo #2 is the card in question; each card drives a single nixie tube.
The mystery diodes appear to be silicon, as they test at around 0.6V forward voltage in circuit (using DMM diode test). Photo #3 is a close-up of the diodes:
I've done a bit of a burn-in test and the 1N4148s don't appear to be getting warm at all, so I'm going to go out on a limb and say it's probably OK. There doesn't appear to be anything in the way of large reverse voltages either, so that's good. Thanks for the responses though!