"E" marking after the production year of the Feldfernsprecher 33

[csmagdo]

Hi everyone!

I have a Siemens (Siemens Apparatebau) FF33 (Kriegsmarine version). Its marking line is: "10324 S&A 1940E".
Does anyone know what the "E" marking is after the prod. year?
It is common this marking at Heer/LW versions as well.

Tnx: Csaba