Probably not much help to you, but the 1988 Rockhopper has 126mm between the dropouts (I measured). Can you not measure them or get them measured if you're not the owner, or is that a silly question?
Thanks for the replies. I'm not the owner, and I'm having trouble getting the owner to measure. He has a wheelset off a 90 Rockhopper which I'm thinking of buying off him for my 89 Rockhopper (which is 130mm).
ps thanks for the offer brocklanders but I'm miles away from Skipton.