>>>  so is the bus going to the main board a 32 or 64bit path?
>      ...
> After some more consideration, I'm guessing that the main board was only
> 32bit.

People who are familiar with the evolution of Personal Iris, Indigo
and Indy product family would have some clue about that.  Sillicon
Graphics dumped all of R3000 products and switched whole to R4x00.
R4000 upgrade was done by entire PCB replacement.  However, the
hardware subcomponents of Indigo/Indy followed continuous design
modification path in priciple, probably, to maintain SGI's awesome
software product assets.

IRIX eventually evolved into a stable 64bit OS which runs 32bit
applications, then NetBSD could do.

Tohru Nishimura