It was a paper shell, plastic’s were just coming on the market.
The standard formula was a measure of flake propellant, two fibre wads probably 3/4 - 1 inch in total, and a measure of shot. We skipped one wad and added extra powder and shot to make up the space.
The loader had 4 stations as I remember it.
We also had a lot of fun shooting the primer’s against brick walls with our wrist rockets.
The reason for formula’s for reloads is to prevent overpressure.
Even minor changes in weight of shot or bullet with fixed powder load can be fatal.
This is an outgrowth of the days of Proof Testing guns. Back then they used black powder, harder to over load charge.
With modern powder, precision scales are a must.
Just adding shot and powder both is very dangerous. You discovered that.