You should check eBay sold listings, or maybe some other auction sites. I don't know how much info you will find, I don't think they pop up very often.
Serial numbers aren't very helpful with old Sunn gear; is it the wedge monitor that fastens to another one for transporting? If so, do you have a pair or a single "orphan" cab?
Are you trying to sell it? How much would you take for it?