Thanks . I’m curious about the R in front of the serial number too
The random prefix or suffix single letters found on Type 56s are still of uncertain origin, though they were definitely done after original manufacture by either the exporter or importer. I tend toward the importer doing it in order to ensure that the serial number was unique within their records, as required by law.
This excludes the 1959-60 "letter gun" production, which are easily picked out by their features (Factory 26, long lug threaded barrel, lightening cuts in bayonet lug and bolt carrier, side swivel) and that the letter precedes 4 to 5 numbers with the same font style.