i'm currently dealing with the same issue as OP and I am lucky enough to have a few different spindles on hand to compare. I've got the exact same crank and original BB taken from an early Colner. I also have another Ofmega BB with different cups, but a similar spindle, and also taken from another Colner, . Unfortunately both spindles are badly pitted so I'm looking for a replacement.

I measured the tapers today with digital calipers and while I don't have the exact numbers off hand (i'll report them later, but i think it was like 12.3x mm), the Ofmega taper was definitely smaller than Gipemme, Zeus and Campy NR. Eyeballing how far the crank went on the non Ofmega spindles showed about a 2-3mm difference. That isn't much at all. I have good reason to believe that with older Ofmega cranks you could get away with using Gipiemme or Campy spindles. Sadly the Gipiemme spindle is also pitted (!), the Zeus is NOS and a weird width any (stamped 55) and the NR is for 68mm. So, my search for something compatible continues!