Hello, there isn't a whole lot of info on the web for Korean coins. I haven't seen anything exactly like this, including the series number on the right. It also looks so sharp it might be a seed or mother coin, which I don't know very much about. Any help is appreciated!
I'm no expert on Korean coins by any means, but, here's some info that might help. I believe its a one Mun from 1742 to 1891. The bottom symbol in the second photo is 42 (1842?) The upper figures either mean P or HUN.....something to do with Military Training Command. I can't find the symbol on the right either, which is supposed to be a series number. The symbols on the front mean Sang P'yong T'ong Bo. The regular series were cast, so, I agree with your assessment that it's probably a seed, mother, or master coin since it has good relief and is polished.