This isn't position 66R2L, the "GENTS" instead of "CENTS" variety. I actually sold one on cover last week on
ebay. The 66R2L double transfer is visible across the entire label block. I think this stamp just has some foreign matter embedded in the paper.
Plate 2L stamps are not that uncommon, it is the second most common plate, but it is 100 times harder to find any one particular position. So, 66R2L can be tough to find, especially a really nice one.