> I need four files, only one of which is OS specific.  Is that correct?

Yes.

> So I could get them, say, from any subdirectory on the ftp server that
> holds Unix files, and then replace the VM with the one you send me --
> correct?

Yes.

FWIW, I have created an SGI distribution, similar to the other Unix
"distributions", containing symlinks to the four required files:

   ftp://alix.inria.fr/pub/squeak/unix/distributions/sgi-irix-5.3

There is a README in the "distributions" directory containing (hopefully
infallible ;) instructions on downloading.  The above directory will
appear on other sites if/when their mirrors notice it.

(Thanks again to Andreas for providing the SGI binary!)

Ian