The rifle selector is on top and there are 3 cocking indicators. I’m thinking that button may be a safety or decocking button for the rifle barrel. More likeLy the former.
Has a tang aperture sight that pops up and is adjustable.
12 ga over 11 mm Mauser. Exc bores. Unusual trigger guard with lever release is the cocking & opening lever.
I bought it from an estate. If you’re interested PM me for pics & more info.
Very nice tight drilling.
