I own a .275 H&H. It was my dads rifle and he had it built for him shortly after WW2. He was a guide and outfitter in the Boulder Basin/Boulder Rim country of northwest Wyoming. He had previously owned many different rifles of many different calibers, from a 30WCF to a .375 H&H. When we relocated from Casper to Denver in the late 60s, he had about a dozen elk and many deer in the B&C record books. He probably killed the last legally taken grizzly bear in the state, and almost all of these animals and countless pronghorns and others with his treasured .275. He told me and many others that "it was the killinest thing I have ever held in my hands".
I am trying to find any recent reloading data for this cartridge, but did want to chime in on the cases. Dad made his cases from the 300 H&H cases, not the 375s. Given the effort that it takes even in modern presses to neck down the 300 H&H cases without ruining them, I can't imagine using 375s. Additionally, I think the .288 caliber bullets that were mentioned might be a misprint or something like it. The 275 uses the same .284 diameter bullets that the 7mm Remington uses.