It is a pre-86 form 1 yes.
The maker is the individual who it is registered to. The dias is engraved with the last few digits of the SP1's serial number followed by "WW" (registered owners initials), and city + state, per requirements of the form 1.
No modifications to the rifle itself.
The guy bought a dias, had an SP1, form 1'd it for registration, installed dias into the SP1, shot it once or twice and never touched it sense.