You can use it....

If the plate is indeed RS 500 then that is classed as a vintage dateless plate. My dad has YRK *** on his car, he used to have it on an old X reg Audi too.

If the plate is R5 SOO then you can't use it as it's the newer style plate (well new up to 2001 or whenever).

But RS 500 is fine on any car - the plate is totally dateless and not one of those cheapo A4 GRP jobbies.