If the purpose was cosmetic there would be no need for the bottom one.
Are you sure there is one on the bottom? The ones on the sides of the ballards only go to the forearm stock line. And on the only one I have examined there wasn't a flat on the bottom.* Also the ballard book talked about the 3 raised flats.
I also see that Dutcher, in his book, refers to the flats as decoration on the Rigby muzzleloading target rifles.
* - I should clarify or I will get ripped, yes there was also a small protrusion on the very bottom against the receiver but I do not refer to it as a flat
