This phone is the most common model of Ericsson wall telephone, called as commonwealth also.
Old category number is 345 (AB535), serial number 120935 from year 1897.
The phone looks now quite different than the time I did get it as a present from my wife.
This was my first old phone !
Phone was missing the writing table milk class and it's frame, crown, all decals and the microphone was wrong type. It did need a lot of gluing, polishing and making parts before I could put it to the wall.
The decals for the bell cover where made as special order for me, from my own taken pictures that were sharpened by the computer picture handling.
This phone has microphone type FG 150 (1897).
It is coal grain microphone with steel membrane.
There is a oilcloth protector for membrane.
Are you missing the protector, do like this.
Get silk (used for silk painting) and stretch it to frame and glue the paper ring to it with bone clue.
Then wet the silk with boiled linseed oil.
Let it dry several days.
Do this as many times as needed, at least 4 times.
Now you have water tight protector for membrane.
Clue the printed paper tag to the middle.
Cut around the paper ring.
Same way were done old water tight cloth's (from cotton) what for example fisherman's used.
Be careful with linseed oil, it is self combustible when drying in closed environment !!!