In the late 30’s ATEA developed a new series of telephones. Although the wall model and desk model used the same electronics, they were very different on the outside and were even made of different materials.
Both versions were in production long after World War 2 and mainly sold for private installations.
The difference between the model 50 and model 51 is the earthing button, which the model 51 has and the model 50 has not. There are white and black versions of this telephone.
The desk model is made of a zinc alloy called Zamac. When this telephone was developed ATEA was owned by the US company Automatic Electric and it is no coincidence that this telephone resembles the AE 34 and AE40. During the German occupation of Belgium ATEA produiced these telephones with Siemens markings for while. ATEA was placed under Siemens supervision during this period.
After the war the desk model was used as a basis for the standard telephone of the RTT, the RTT 56.
See also the website of Jan Verhelst, ATEA historian, for more information on this phone: Het merkwaardig verhaal van een ATEA telefoon (in Dutch)
Or see here for the same article in English.
The wall model was made of bakelite, even with a bakelite back plate. While the desk version has a more classical configuration with all components mounted on an inner frame, the wall version is more modern with all components mounted on the back plate.
Although my example was made in 1942 and has a capacitor made by Siemens & Halske, it has the ATEA logo at the front. I have never seen a wall version with Siemens markings, like the table model.