Thanks to Jonathan Stone and "Maximum Entropy", as well as Simon Burge
and Gregory McGarry for their prompt replies on this subject, almost all
of which suggest that I am hopelessly behind the times.

I tried to get 

ftp://ftp.netbsd.org/pub/NetBSD/arch/pmax/network-install/diskimage.tar.
gz

but the symbolic link is broken (that kinda stuffs that ......)

So, I tried 

ftp://ftp.netbsd.org/pub/NetBSD/arch/pmax/32M-diskimage.gz

and got the results I posted.  Saying 1.2G is "no longer supported"
still doesn't explain why it doesn't work on my RZ23 (but since my
posting I've tried an RZ25 and it seems to be bootable - now to see what
I can do).  That kind of implies it NEVER worked.  Since "Maximum
entropy" posted a recent (12th Nov) message saying he was having trouble
with RZ23 on 2100 machines, I'd say there is some inherent problem
here......

What about some minimum disk size recommendation, as well as some "I
started with Ultrix 4.x, how do I get it going.... ?" documentation.  I
will gladly help if and when I can - nothing worse than a fence sitting
critic (which I don't want to be)

I will look at 1.3 Alpha to see what I can see.  Luckily I have a SZ12
external expander, so I can run more than 2 disks with relative ease.

Regards

Jason Armistead