GpSnoopy is on the right track -- it is definitely a "Fashion" button, made for fancy civilian-usage coats/jackets.
Another ID-clue, in addition to the design on the button in GpSnoopy's photo:
"Fashion" buttons were (and still are) often made with a black paint coating, for a different "finish" than shiny brass... and we see remnants of black paint on your button.
You asked if your button has a British crown. Actually, it seems to most closely resemble the Empirial Crown of the Prussian (German) Empire, from the 1800s through the end of that Empire shortly after World War One.