the seller was writing that he doesnt realy know what for a caliber it was and dont give any gaurantee on this . to my very limited experience with such stuff I am quite sure its the 28/40, 28 bore case 40mm long. believe it or not.
the Lefaucheux-type action was used on cheap guns up to the great war in germany but ass an old Fuchs you have notice that the stock design was much older and a little bit outdatet when this büchsflinte was made. I would say its made between 1870 and 1885 but you see such stocks on older muzzle loading hunting rifles.
probably made for a very conservative gentleman.