Rim measurements from drawings

375 H&H Belted per Kynoch drawing:

Thickness: 0.050 inch
Diameter: 0.532 inch

375 H&H Flanged per Kynoch drawing:

Thickness: 0.060 inch
Diameter: 0.572 inch

375 Flanged Magnum per ICI Metals Ltd drawing

Thickness: 0.060 inch maximum
Diameter: 0.572 inch maximum

Rim measurements from samples
(British manufactured cartridges)

375 Belted Rimless Magnum:

Thickness: 0.042 to 0.055 inch
Diameter: 0.527 to 0.532 inch

375 Flanged Magnum:

Thickness: 0.052 to 0.060 inch
Diameter: 0.557 to 0.572 inch

375 2-1/2 Inch Flanged Nitro Express:

Thickness: 0.059 to 0.065 inch
Diameter: 0.520 to 0.528