The fellow in that video has his dates wrong. The shotgun slug was invented well before 1931.
"the slug didn't come out til 1931" (Brenneke invented his slug in 1898. The Foster slug was invented in 1931. Found that out over at the PAFOA forum.)
and they were using these shells during the depression". The implication there being that the Depression occurred before 1931. Maybe a little bit of it did but not most.
There are at least two other threads on other fora discussing this at present.
PAFOA and Graybeard Outdoors.