Chuck Hawks tends to perpetuate misconceptions in his articles sometimes.
Holland and Holland actually introduced the belted case in 1904-05 as the 400/375 (375 Velopex) which is of course a totally different cartridge than the 375 H&H Magnum.
I have read that the 275 belted magnum predated the .375 magnum by a year as well but am not sure of that.
Also, "Express" was not "a British term used to denote a more powerful version of a cartridge" but instead denoting a loading of a cartridge with a lighter bullet at higher speed than a standard load.