I'll throw my thoughts and experiences in on this one.
1st Drews pressure data. The SAPL after 12/50 should alert us to something unique to this pressure. A click on the link provided Shows cartridges 12/50 with a note "FUN tir cartridges are a 12/50 caliber "gum-cudgel" type cartridge that can only be used in products from the SAPL Co. I translate gum-cudgel as Rubber Club. They look like non lethal defense products firing a rubber ball.
2nd GLS comment Pretty much all new production guns are finished "In the White" with some polishing afterwards.
3rd Ted S/Imperdix" comment are I'm sure the answer.
A wall thickness survey before and after the thin spot,particularly adjacent to but up against the ribs would most likely tell what the original thickness was.

I once sold a 1935 H&H 12-2" Dominion back to H&H New York.So as not to waste time doing the Wall Thickness song and dance I advised Guy D. The wall thickness is only .014" each Barrel. 9" back from the Muzzle. Without looking up from the gun he said "Don't worry that's the way we made them!
Hope some of this helps. It's a Sleever for sure, good lock finding suitable tubes.