If I am correct, the .35 slug was in a 30-30 casing. The slug was 200 grains. It wasn't much good beyond 150 yards, but it would plow through brush and immediately drop the animal that it hit. And, they didn't move or kick. The effect was immediate.
As a side-bar, I remember reading that the Texas Rangers used them as a saddle gun in particularly brushy country.
Your description of my old and very reliable Marlin lever .35 Remington is quite accurate. As a deer gun in Central VA it is quick handling and invariably effective.
I would love to have the semi-auto version you mention but have not seen any...