Rolex has produced only two different models of moonphase wristwatches, references 8171 and 6062. Both models are automatic, however reference 8171 has a snap on back, while reference 6062, the Oyster version, has a water-resistant-type screw back.
The reference 8171 is considered very rare as it was produced in a small series between 1949 and 1952. It was made in stainless steel, 18K pink or yellow gold.
An example of this model, reference 8171, is illustrated in The Best of Time - Rolex Wristwatches by James M. Dowling & Jeffrey P. Hess, p. 209.