The R means Reissue and the number means the last number in the year. (i.e.: R9= '59 Reissue, R0= '60 Reissue)There's no such thing as a R1, because the Gibson Les Paul didn't come out until 1952. I also think there's no such thing as a '53 Reissue or a '55 Reissue. The reissues are the R2 ('52 Reissue), R4 ('54 Reissue), R6 ('56 Reissue), R7 ('57 Reissue), R8 ('58 Reissue), R9 ('59 Reissue), and R0 ('60 Reissue).
56 were gold tops with p-90s
57s were gold tops with humbuckers
58s introdued the flametop with humbuckers
59s had slimmer necks and bigger frets than 58
60s has slimmer necks than 59, and i think plain maple tops
i dont know any of this from personal experienc though