No, rock salt shells are real. I've seen them - but I think they were used mostly in the 50's and 60's (maybe earlier as well) for basic home defense in rural areas. There may well have been a company that made and sold those loads, but I don't know that. I'm pretty sure the only ones I've ever seen were hand-loaded.
I don't know much about tinkering around with a shotgun shell, but I have the impression from listening to old timers talk about them that all you did was open the shell, pour out the bird shot, fill the cavity with rock salt, and seal it back up with some wax or something. They weren't intended to be lethal. The primary purpose was more for getting rid of a nuisance - like the kid that is sneaking into your garden and stealing your tomatoes at night. You don't want to kill him, but a posterior full of burning salt is a surefire way to keep him out of your hair for a bit.