Brodie,
The lens is the R Summicron 35mm f2.0 lens. It is the last version of this lens design for the Leica R system. The E55 relates to the 55mm filter size. These lenses were available in the 3 Cam and the ROM versions (I am not sure if they came out for the 2 Cam version). The latest of the these lenses came out in the ROM version after the introduction of the R8. The ROM is only usable for added information to be provided between the lenses and the R8 and R9 cameras. A ROM lens will not fit the Leicaflex manual cameras as I recall which includes the original Leicaflex, SL, SL2, and SL2MOT. They will fit the Leica R3-R7 which were based upon the designs of the Minolta Camera bodies including the XD11. The 2 and 3 CAM versions of the Leica R lenses I believe will fit all Leicaflex and Leica R cameras.
Rich