Sutherland, C.H.V. The Roman Imperial Coinage, Vol. I, From 39 BC to AD 69. (London, 1984).
The rarity ratings in the early RIC volumes are inaccurate. They were heavily influenced by Cohen's prices for the same coins, however, Cohen's prices were influenced not just by rarity, but also by collector demand. Interesting, desirable types are, of course, priced higher than ordinary or less interesting types. Some very rare types are rated only scarce in the early RIC volumes because the authors incorrectly believed an inexpensive type could not be rare.
C: Common to very common
R2: [Rare] 11-15 known [in the collections examined]
R3: [Rare] 6 to 10 known [in the collections examined]
R4: [Rare] 2 to 5 known [in the collections examined]
R5: [Rare] Unique [only one in the collections examined]