There has been much controversy about "dimensional differences" between the 8x50R (M93) Austrian, the 8x53R (M88/90 )Austrian , and the 8x50R (Type 45--M1902/3) Siamese.
For all practical purspose, Siam bought from Steyr in 1897-8, a batch of Mannlicher M88/90 or M90 Rifles and carbines And bayonets, the left overs of a defaulted Bulgarian Contract (The Bulgarians then acquired M95 rifles and carbines in 1903-1914);
Whichever of the Austrian cartridges the Early Mannlichers were chambered for, the Ammo was interchangeable. (M88/90 and M93 Cartrridge )
Siam then in 1899 contacted Mauser for trials rifles (7x57 M1898/1902 design) as well as a possible "8x50R" type Mauser rifle. Mauser at the time had just released the M98 design, also in a Sporting Version Action for British Rimmed Cartridges, and designed a "slanted" magazine-well Rifle, with a Small ring diameter reciever, but M98 Three lug Bolt design, and Long Firing Pin ( as in the Commercial M1904 and the Turk M1903), and with the M1902/1904 Commercial rear sight.
As Mauser Oberndorf was fully occupied with both German Imperial orders and the Turkish Contract, as well as the M1904 Commercial, they Licensed out the manufacturing to the Dai Nippon Arsenal ( Export arm of Koishikawa Imperial Army Arsenal), to manufacture the design, along with any improvements that the Siamese may have requested.
The Forward moving Bolt/receiver cover ( also used on the short-lived Japanese Type 35 rifle (between the T30 and 38) may have been a Japanese Suggestion, for the problems of Tropical use. Why forward moving and not-Bolt-handle-actuated, I don't know...maybe because the forward sliding was a "silent" item (the Bolt cover on T30/38s was a rattler, and in practice, usually discarded in combat).
As to the ammunition, and the Rifling/Bore relationships, they followed Austro-Hungarian practice... ( Bore .315, Projectile .322-324 Flat based; Grooves .328-330, so that "Base Upset Obturation" of the long cylindrical bullet allowed for a driving band to gas seal, and the rest of the long jacket barely cut by the rifling, and so reduced jacket-to-bore friction and Metal fouling. ( steel jacket bullets used, with or without Ni- plating.)
The slighly different dimensions were probably a function of extactor design differences, and tolerance clearances differances due to being used in a Camming turnbolt rifle rather than a Straight-pull action with less "camming" extraction power. The relationsip to the 8x52.5 (aka 53)R Murata is tenuous at best, the Murata case design having been obsoleted by the smaller 6,5mm Type 30 cases. Japan's last use of the Murata was in the Russo-Japanese War, but usually as a second line arm only ( non "attack" units). Front line infantrey were armed with T30 rifles in 6,5mm).
The original T45/46 (Mauser) design rifles are Identifiable by the complete ramps of the M1902 patent rear sight, with full Siamese numbers (digits) stamped there-on (similar to the 6,5mm M1904 Vergueiro rear sights).
After WW I, with the Observations gleaned from Siamese Officers who had been attached to the French and British Armies in France, it was decided to upgrade the ammunition to a Spitzer bullet, for both MG and rifle. In a financially straightened atmosphere, and bolstered by the New King's formation of the "Tiger" Corps with BSA .303 rifles (Type 62, 1919) using spitzer .303 bullets, the Siamese, in conjucntion with Koishikawa, decided on a new cartridge, being a slightly larger 8x52R cases, with a 168 grain flat based bullet of .324 diameter, to function in the re-chambered 8x50R barrels. The OAL of the new T66 (1923) cartridge was the same as the original T45 cartridge, so no magazine changes were required...also, the Rim diameter was also identicxal to the T45, so no Bilt face adjustments or extyractor adjustments required, ands the Body dimensions were increased slightly, to allow for a New (T66) Chamber reamer to fully "CVlean up" the existing chamber from Base to Mouth; that way, the T45 case head (.490) was increased to .500-505 (T66); the shoulder was moved forward slightly to change the angle wrt the Austrian case; and of course the extra length also cleaned up some of the eroded throat of the rifling, still allowing for the Base Upset to occur in the normal fashion.
The rear sight ramp was re-cut (or precision ground) to reflect the flatter shooting Spitzer Load.. All this conversion work was done in the recently modernised RTA (Royal Thai Army) Arsenal at Bung-soo, just outside Bangkok. The Country's name had been changed from "Siam" to Thailand" some time in this period but the change was not universally adopted until after WW II.
At the same time, Siam had ordered a New Rifle from Japan, based on the Arisaka T38 Action, but in 8x52R; due to then 1923 Kanto Earthquake, wihich damamged the Koishikawa factory (Near Yokohama, down from Tokyo, the Delivery of the T66 (Arisaka) Rifles dragged almost to 1930. In typical Japanese fashion, the T66 was fully engineered to be non-interchangeable with the T38 (Japanese) Rifle, althought he design was the same; Only the bayonets were interchangeable. Siam also was buying Surplus T38 rifles at the time (in 6,5mm).
For the MGs, it was a simple case of substituting the old barrels for new ones (Vickers mostly, after WW I) and recalibrating the Sights. Siam continued acquiring ammo from Japan, began making their own, and after WW II into the 1960s, had Kyb\nock make 8x52R ammo and empty cases for them for loading in Bungsoo.
A very intersting cartridge history from a much under-reasearched Country.