I honestly don’t know, these are all pieces Iv put together through old notes, memories as a kid, stories past on and news clippings. A lot comes from my 72 y/o dad who remembers most of it, but is a bit foggy. I believe the Taylor came to be right around that same time, and I believe is based off the .458wm so good chance he caught wind of it and wanted to try it. The barrel is stamped RCBS 375-338, with no mention of Taylor on it, but the brass that came with the rifle is stamped .458 Wm, and also some stamped .375 h&h. Looking at photos online, it looks a lot like the Taylor.
Something I noticed with my fired brass though, is the shoulder looks sharper on my fired brass. I’m thinking they haven’t been fire formed yet maybe. These are from .458 Wm brass.
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I would say you are exactly right. The ones with “rounded” shoulders have just been run through the sizing die and haven’t been fired yet.