Mexican Onza question

1977topps

I purchased a 1979 MO Onza graded MS65 by ANACS a while back. I cracked it out and sent it to PCGS, where it also graded MS65. However, they list the variety as KM-M49b.3, which was not on the ANACS slab. I can't find this coin in the 1901-2000 World Coin catalog. Can anyone tell me the story about this variety? Thanks for any help.

Steve27

M49b.3 Type 3 Rev: Left scale pan points to "U" in "UNA."