The answer to that is because I've got one and in fact now have the cover off, so to confirm (and I was wrong first time).
Rail 1 - gain stage
Rail 2 input buffer, tape buffer and powered input boards if any.
and as an added bonus I now know that the nos 20KAX2 pot I bought from spiratronics is exactly the right thing and will now get a 10KAX2 for balance and replace both - which looks easy enough to do.