It's hard to tell from the angle, but it looks like a British style crown. I can not locate the NRM in my data base. Most, if not all Royal Navy and Marine buttons will have some kind of anchor device. In my opinion the N represents an English county, the R means royal, and the M means militia. Although many of these volunteer and militia buttons have been identified, many have not.
I would do some research and see what regiments were in the area where the button was recovered.
Hope this helps some.
God Bless
Dan