I'd be interested in why you believe it's a "prototype" but have no idea of the value.
If it's a prototype, there's likley no VIN, because it was never built to be registered and be put on the street. If it has a VIN, and it's a prototype, it won't be a normal VIN.
If there are 8, then it's likely not a prototype. Prototypes by Harley were rarely if at all, sold to the public, rather retained in the museum. As well, First Year productions were known to be limited, so 8 of a specific color/options isn't rare. Back in 1977, HD also made the FLHS, which was a stripped FLH bagger, which was the pre-bike to the Fatboy.
The FXR was first released in 1982, so in order to be a prototype, it would be titled earlier. Bikes were already being built and sold in late 1981 as 1982's. It is also a shovelhead, as the FXR never saw an Evo engine until 1984.
Jerry Branch of Branch Performance built an FXR bike for racing back in 1984, but it wasn't a "prototype". It was raced and campaigned to promote the business and used for R&D and horsepower mods with the newly released Evo engine. Put out about 102 hps.
The only model in the FXR line that's rare, was the 1984 "Disc Glide" (FXRSDG) which offered the first chrome package, special paint and other options not found on other bikes for that time.
Hey....I could be wrong about all of this........
“When the going gets weird, the weird turn professional....” ~ Hunter S. Thompson~
R.I.P. Rubberdown. 1971-2012