MP 40 identify

[Snap552]

My father bought a dewat MP 40 years ago.I have it now.Its in almost unused condition as far as I can tell.All numbers match except for larger stamped numbers on the rear cap.its stamped on the rear cap MP 40,ayf with what looks like an over stamp from 42 to 43.it also has the letters COS stamped with the number 3968.it also has many waffen stamps all over the gun.Any suggestions?

[Petroleum 1]

The AYF means it was made by Erma. The cos marking is Merz Werke a subcontractor. Is it registered??