It was called the MP40 when it was originally added. This eventually became a placeholder name when it was decided to be replaced with Maschinengewehr instead.
When it was called the MP40 it was the only weapon to have numbers used in its name.
There was an error in the description that says it is "chambered in Calling Card ammunition." It should actually have said that it was "chambered in MP40 ammunition." Update 188.8.131.52 changed the description to say it is "chambered in Maschinengewehr ammunition."
It is based on the MP 40, a submachine gun developed in Nazi Germany.
The Maschinengewehr also has the former part of its name based on the real-life equivalent.
"Maschinengewehr" is German for "machine gun," even though the Maschinengewehr is a submachine gun.
"Maschinenpistole" is the German word for "submachine gun." The literal translation of this is "machine pistol," referring to the pistol ammo used in submachine guns. The real-life equivalent, the MP 40, is an abbreviation of "maschinenpistole 40."