Looks like a 1992 R500 to me. Have a look at the 1992 catalog - the 3.0 series frame decal on the chainstay and the made in USA seat tube decals are a match. Should have had a model decal on the top tube, but perhaps they were out of them on the day your frame was made.
What do you see that makes you sure that it's not the original the fork? If I have the right model and year, the original fork would have been painted white. Any chance it's just been stripped and polished?