How many Swingers w/ sunroof were made in 1975?
Do you have pictures of the sunroof? A lot of aftermarket sunroofs were grafted in over the years, might not even be original.
If it is original, it would probably make it a somewhat "rare" car, in that it probably wasn't a popular option. But, don't mistake rare with valuable. On a '75 swinger it's not going to do much, if anything, for resale value. And I doubt if there's an official number, very little info is out there regarding production #'s on specific options.