I can't let it go. I have been doing more research and I have come to the conclusion if I treat the .375 Flanged Magnum NE as a different cartridge from the .375 H&H belted Magnum and make sure my barrel is marked accordingly I am on safe grounds.

I am correcting the headspace on a 577/450 Sporter Martini right now. I have the barrel in the lathe waiting for the reamer to come from the rental place. As soon as I get that job done the .375 barrel goes in the lathe.