Does anyone know how ammunition factories roll crimped their shotgun shells back in the day? Surely not the same way we reloaders do.

Considering that factory buck shot, and even some slugs are rolled crimped, nothing has really changed on their set ups, and still have a spinning tool head roll crimpers in the tooling line up to roll crimp the shells during that production.

Hell, even in standard fold crimp hulls, their final taper tooling is still spinning tools that will apply a little heat to the top of crimp to get that deeper inward taper than you can with just downward pressure alone.

Skip to 4:30'ish to see the loading of shot shells.
https://youtu.be/5XhOSXD-VKE

Also, keep in mind that the production line tooling is set up to load a gauge at time, Hence a great deal of shells for that time frame production run in that gauge, before they will change the tooling to load another gauge for it production run next. So no, they do not have 4 different production lines for all the different gauges, but just a single production line that the tooling on it is swapped as needed.