Alright!!! I had to rely on the "Mustang God" for this one. I personally have never heard of such a thing. But this is what he had to say and I quote......
"I have heard of a Capri RS but not a Mustang RS. That does not mean that they don't exist though"
In other words it could be one of those rear dealer mods that were considered stock. Sort of like a California Special or a High Country (same car, one from California the other from Colorado). It may not have been that popular or they may not have made to many of them.
They world my never know.....
Shelby