The following is from P.O. Ackley’s “Handbook for Shooters and Reloaders”, Volume One:
“The .375 Durham Magnum is the .338 Win Mag case necked up to .375 with a 35 degree shoulder angle to provide a very fine short .375 Mag cartridge with an overall length suitable for use in the standard actions. Ballistics is practically the same as the standard .375 H&H Mag. (This book was first published in 1962!). This is virtually the same as the 375/338 Belted Newton.