I bought some supposedly 378 Weatherby Magnum brass on gunbroker.com some time ago. It turns out it was 30-378 Weatherby Magnum brass with a 378 Weatherby Magnum head stamp. I presume someone bought 378 Weatherby Magnum brass and necked it down. I was planning to resize it to 378 Weatherby Magnum brass again but have lost interest in that project.
A few pieces of said brass are brand new while the rest is at least once fired. Since I didn't buy the brass new, I don't know how many times it would have been fired but, since some was still new, I'm thinking probably once fired. If you're interested in buying the brass, PM me and I'll come up with a price for you.