7.65x53 Belgian Mauser, 7.65x54 Argentine. Did the Argys have a reason or did they just want to be different? I think same question applies to the x54R Russian and the x53 Finnish version.

If I an not mistaken it is because the actual size is 53.5 mm. One rounded up and the other down.

Rad

The proper nomenclature is 7.65 Mauser.
There was a letter from an Argentine ordinance officer in Handloader Magazine several decades ago. He said the Argentines did not use any nomenclature for for the case length.
In any case (very big pun intended) none of the metric designations are very exact in the sense of direct conversion to .0001" inches. If you want to know what the dimensions are you need the original cartridge drawings. They are some what more complicated than those from a loading manual.

The current CIP standard which is also not the original but the best I have on my computer.

Minimum chamber 53.7mm
Maximum Cartridge 53.6mm

I have also found this on the 6.5X53R Dutch Mannlicher cartridge.
The original Dutch drawings are actually on line. This case is actually 53.6mm = 2.110.
The chambers have plenty of clearance at 2.150.
I have never seen a Dutch chamber drawing.
But....

The rimless version -

the 6.5X54 Mannlicher-Schoenauer

The chamber____Minimum Length = 53.65mm
The cartridge ____Maximum length = 53.65mm

53.65 = 2.112

Incidentally the CIP drawing say "Dimensions for Proof Barrels".



FWIW