C-C, there is some confusion on the 79 and 81 proof mintmarks. There is no filled MM on the 1981. Picture of 1979 filled MM.

Some time during the year they changed to the sans serif MM.

This is the type 2 for 1979.
In 1981 the type 1 MM is the same style used for the 1979 type 2.
Again, sometime during production, they changed to a serif style MM. This is the type 2 MM for 1981.

Hope this helps.