The M is believed by many to stand for "membrane" but I don't know of anything official to back that up.
For the F, many believe it stands for "flipper" which is the informal name for the pivot plate mechanism using in the buckling spring. Again, I know of nothing official.
I think the F might also be a nod back to the early IBM typewriter model naming scheme (A,B,C,D) but I don't know of a model E designation.
Some call certain terminal buckling springs Model C's but they're really F's that had a C<number> designation due to the hardware they were designed to be connected to.