Dealt with all the active rust, serviced the inside (so much gunk packed in there...) and gave it a sympathetic polish. I don't think I can do anything about the patchy nickel, which was already rubbed thin in some places and covered in microscratches, but it does look a damn sight better.

I'll probably be returning it to him tomorrow along with the knowledge of its model and window of production. Thanks everyone for the insight and especially thanks Handejector for the information about the finish.