Thanks to Brian I stumbled over a US company named Electronic Systems International Inc. from Lancaster, PA.
They made 40x46SR cartridges (M433) or components for and known activity is at least in 1981.
The only reference I can find in the web (in relation to Lancaster, PA) is a company named Wacor Electronic Systems, Inc..
They might be the same company or part of it or a successor or also being totally unrelated.
Also I am not able to find any of the two in the US 1999 manufacturer code list.
Anybody out there who knows more?