Black KM button

[Christophorus]

Hi to all,
I need your help as regards this KM button lacquered in black, the rear appears to have lost its paint.
It's not the first time I see this type of button (black) and I would like to know your opinion about it.
Once I was told they were worn by officers on german submarines and was given the following number, corresponding on an Assmann reference??
Model 281062596978

Any help would be appreciated
Christophorus

[Christophorus]

Another photograph to better illustrate that these types of black buttons have existed:

DR F & Co - Kriegsmarine - 1939

Any suggestion - idea?