The search for the lost Cray supercomputer OS" by Signe Brewster on GIGAOM:
From: John Hogg
Subject: Resurrecting Cray OS
Date: January 14, 2014 7:32:09 PM EST
To: Mike Alexander, Jeff Ogden
Looks like MTS has been much better preserved than Cray OS.
The search for the lost Cray supercomputer OS
From: Mike Alexander
Subject: Re: Resurrecting Cray OS
Date: January 15, 2014 12:55:47 AM EST
To: John Hogg, Jeff Ogden
Yes, indeed. In the process we've preserved more than just MTS. We've been able to give source back to the people who wrote it a couple of times because it was in the MTS distributions even though they lost it themselves.
I was in the Science Museum in London recently and had a chance to sit on a Cray 1. They have only (or mostly) just the CPU, but it looks fairly complete. It's still an impressive machine even if it is slow by modern standards.