Post WWI up till say 1933 when there was a name change to Waffa, Iver Johnson Sporting Goods Company handled what looks to be the whole line of Simson's wares, using their same Modell Nr. Simson was heavy into racing(Otto Reif, Hugo Vonderlindetc.) & was attempting to capitalize on the pigeon shooting craze at the time. Iver Johnson Sporting Goods Co. carried O/Us and others. I think them to be common but then again I'm up to my eyeballs in German info. Backbored, overbored, underbored, I think it was happenstance that if the parameters add up to that. I'd invite Ford & Baumgarten's opinion, but I don't think one would want to throw a solid projectile down the tube in an overbore/backbore. Germans centered their gunmaking on the 16 bore platform, barrel harmonics for multi-tube variants and they could turn raw tubes out day after day at those spec. They seemed to be cautious on the 12 and 20 bores, and even the 24, as they were usually 1 bore more or less from the nominal. Ford can put us on the straight and narrow here. I can't see if there is a constriction mark on the right tube but Simson did go the extra mile and have it Nitro proved.

A bit more stylish version from the same period:
http://www.vintagedoubles.com/catalog/?sort=4&process=fullview&gunID=1747

Kind Regards,

Raimey
rse