1956 Nickel. M. M. Doubled Die .

Not a doubled die.
It is a DDD ( Die Deterioration Doubling) with the doubling moving out towards the Rim. The coin is in a Late Die State and all your 39 coins were struck by the same Die.

What is N.M.M.?
John1

The Philly mint used the mintmark on the War Nickels on the 1942 type 2 - 1945.

But the other coinage didn't start until 1980 with the 'P' mint for Philly. (only cents did have the 'P' mintmarks except for the 2017-P cent)

So not having a mintmark from Philly was never really an issue. (except for the 1922-D with the over polished dies. Philly struck no cents that year) There were some years where the mintmarks were left off on the San Francisco coins and also one year on business strike on dimes from 1982 Philly.


A few Cents has the missing Proofs on the post 1980 cents:


A few Nickels also missing a 'S' mintmark:


Probably more of these, just don't see them on my image host right now.