Third Reich uniform button, large 25mm size in zinc.

This is a German Third Reich uniform button,  standard pattern Hoheitsadler (National Eagle) button, with the eagle clutching a swastika wreath, which was the national emblem used across virtually all Reich organisations from 1935 onwards.
The maker’s mark:
Stamped on rear is the “A” for Assmann & Söhne of Lüdenscheid, one of the most prolific German military button and insignia manufacturers of the period. 
The “M25” marking for 25mm button. 
The challenge with this eagle pattern is that it was used across multiple organisations. The key distinguishing factors are:
∙ Finish – this appears to be a grey/silver aluminium or zinc alloy (Feinzink) finish, consistent with Heer (Army) uniform buttons from the late 1930s/early 1940s
∙ Eagle style – the spread wing angle and the relatively compact wreath below are consistent with Heer rather than Luftwaffe or political party variants
∙ Size – 25mm was typically used as a tunic front button
Possible branches to investigate:
The most likely candidates in order of probability are:
1. Heer (Wehrmacht Army) – most common user of this finish and size
2. Political uniform (NSDAP or affiliated organisations) – though these often had slightly different eagle styles
3. RAD (Reichsarbeitsdienst/Labour Service) – also used similar patterns