I used to load some zinc 12ga. cases years ago. I think I used an old MEC 250 single stage loader, if I remember correctly. This was even before plastic wads were used. You made the wad column out of fiber wads of varying thicknesses. The shot was covered by an over-the-shot heavy paper disc, and the crimper on the old press was just barely put into contact with the mouth of the zinc case and gave a slight roll inward to secure the shot wad in place. These old presses are basically all the same in operation, and can sometimes be found at gunshows and the like for next to nothing.