M probably stands for Magyar (Hungary) and Ericsson had a joint venture factory in Budapest.
The dial looks German but the telephone does not!
http://www.postamuzeum.hu/en is the website of the Budapest Telecomms Museum, and somewhere on this they have a full set of circuit diagrams of old telephones. But it's past my bedtime, so you'll have to do some searching yourself.
Good luck,
Andy E.