I knew I'd seen one marked with "E" somewhere:
https://simpsonltd.com/erfurt-luger-...te-1920e-1914/
Pictures are not good of the marking, but it looks very similar to the one you noted.
So there are at least two with the 1920 "E" property stamping.
I tried to copy the pictures but cannot, they only copy as a web page.
Maybe someone else with more computer skill can copy one.
Still no Idea of the meaning.