OK so I am by no means an expert but I know stamps in general and I am 90% sure this is one, it would make a recessed mark, which is the only way you can "mark" metals and pottery and the like back then. Are you sure it's brass? By the look of the man he is very relaxed and almost looks as if his top button is open on his collar, so it is not an upper class item. This would be a working man's stamp that he probably made himself. I am sure at one point in History marks were made simpler than putting them into a spike or handle. So a plate stamp that was pressed or hammered.
The almost "childlike" feel of it just reminds me of the first time I saw a McCoy and I thought it was a flower pot made by a child in school, mind you I was only 12, had no clue, and I was making a ton pottery, I thought mine was better HAHAHA
I don't know but I am sure it is a maker's mark, it seems completely inappropriate for written letters as a wax seal or embossed in the paper. Most of the old seals create a raise and most all are just initials (unless you were rich)
Hope this helps your journey!
