This one is double dated. It's seen on some others, and I think this often happen after 1921 which seems to have been the last time they used the date code within the circular text with Bahco etc., and started to stamp with the separate little circle and the additional number. In this case the old stamp is K for 1921, and then they made the new stamp two years later - M13 in this case. Was the bottom made and stamped two years before they actually assembled this stove? I've seen a couple of similar stampings in the gallery. E.g. this one, but it has an L instead of my M (It should probably read 1922 in the title there). Some crappy soldering jobs done in the past, but I'll leave it as it is for now.