Well as the chart shows, there is no "XR" year stamp for any year. The few years that do use two characters for the year use a matching pair of characters. So, the R in your case doesn't mean anything relavant to the date.
My 1963 700 (6mm) has a date code of DKX (and a five digit serial number) which Remington even confirmed to me in an email that it was August 1963, with the X having no relevant meaning.