I don't think the files that Miller has go back that far...but have you tried Miller? They will want Model and Serial from the id plate if it still has one. Miller had the factory owners manual, parts lists and schematics for mine. I am surprised your schematic does not show all the wiring, mine are very complete including the optional stuff that was available at the time. Resurrecting some of those really old and oddball machines can be tough. That machine did not have a hand crank stub at the end of the crank?
Get a hold of this fella
http://www.youtube.com/watch?v=2DAOEhqVhsIThere is a Hobart boneyard/welding repair place somewhere up north but I cannot find the dang number. Good luck on it.