[jan-jaap]

Not possible. The backplane of the Challenge and Onyx series are entirely different. Even Onyx RE2 and Onyx IR have different backplanes. The Onyx has 3 PowerPath II slots and takes 1x CPU, 1x IO4 and 1x MC3 boards, the Challenge has 5 PowerPath II slots so you can have up to 3 CPU boards (and/or multiple IO4's or MC3's). Obviously there are no slots nor power bricks for graphics in the Challenge.

There used to be a Challenge DM which was crippled to use only one CPU. That was enforced with a magic screw, maybe that's what you mean?

I never had much luck with Onyxes: my first one, an RE2, would eat at least one board per year for no apparent reason. The second one has a busted IR raster manager which kills the desktop as soon as you start using it properly. My Challenge on the other hand never gave me any grief, and with 12x R10Ks it's a pretty fast IRIX 6.2 box.