Purchased from BSA by King Rama VI, a graduate of the finest English military academies.
IIRC, 10,000 rifles in the contract.
When you are the King, you can do things like that!
BSA had lots of parts left from wartime production, so you will find British-marked parts in many of these. It is entirely possible that the King got a good deal on these rifles. They are marked specifically to his Wild Tiger Corps, a rural village militia. Being that Siam (Thailand) is next-door to Viet-Nam, these poor rifles were in absolutely the worst climate in the world for a piece of precision equipment. Many of these show the ravages of the South-East Asian climate, but they survived half a century in conditions which would turn an M-16 into powder in 5 years.
The "Siamese Smellies" represent only a fraction of 1 percent of total SMLE production. They are rare by any standard, their values being held down mainly by the terrible condition in which most are found.
The regular Siamese Army of that period was using a mixed bag of Mausers and special Siamese Arisakas in 8x52R Siamese calibre. The SMLEs, of course, used .303.
A Sandhurst education does pay off, even for a King!
Anybody who buys 10,000 Lee-Enfields at a crack is my kind of guy!