Siam, now known as Thailand, was one of the few independent south Asian nations around the turn of the 20th century. Looking to modernize its military to protect against colonial imposition, is decided in 1903 to adopt a copy of the Mauser Gewehr 98 rifle. Siam (which changed its name to Thailand in 1938) didn't have the manufacturing base to produce the necessary quantity of rifles, so it contracted the job out to the Japanese arsenal at Tokyo (Koishikawa). Between 1903 and 1908, about 400,000 of these rifles were manufactured, chambered for an old-style 8x50R cartridge using a round-nosed projectile. This was called the Type 45 rifle, based on the Thai calendar.

About twenty years later the Siamese military was improving its machine gun armament, and revised its cartridge to a slightly longer 8x52R case using a modern spitzer bullet - as basically every nation did at some point in the early 20th century. The existing stock of Type 45 rifles were rechambered for this new cartridge, and it is fairly rare to find an unmodified example today. The conversion is easy to spot, as it also included grinding down the rear sight ramp to fit the flatter trajectory of the new cartridge. Range markings had been stamped on the side of the rear sight, and converted guns have these marks partially removed by the reshaping of the ramp.

As with all rifles made by the Japanese before WWII, the Siamese Mausers are of excellent quality, although many have led rough lives by the time they reach the hands of a collector today and are often in pretty rough condition. While they make an interesting addition to a surplus rifle collection, they have also long been used as the basis for conversion to .45-70 conversions, as the bolt and magazine are already designed for large rimmed cases, unlike most other military Mausers. This has led to them being rather more rare than one would otherwise expect.