True, not wildly valuable but that's because so many of them are toast.
They were built right after World War One - 1922 - by BSA as a commercial contract for the Siamese Government as weapons for a village militia scheme, the Wild Tiger Corps. The funny writing says that the rifles were the property of King Rama VI and give the Buddhist year of adoption which is, if I recall rightly, 532 years more, dating from the birth of Lord Gautama the Buddha.
There must have been official involvement in the purchase in some manner, because BSA evidently got permission to use wartime British-Army-marked small parts in these rifles, although at least the bolts, barrels and bodies (receivers) all were new manufacture. Some rifles you will find Broad-Arrow-stamped small parts in, but never the big parts, unless, of course, something has been changed. The actual order for the rifles was placed by the King himself, who had been a student at Sandhurst and had developed an appreciation for the simple and practical Lee-Enfield rifle. The regular Siamese Army at this time was equipped with Mausers, but the King's little Militia got the BEST.
There were only 10,000 rifles in the contract, making this one of the scarcest Lee-Enfields ever produced: about 1/10 of 1 percent of total production!
The squiggly thingies on the right-front of the Body and the Barrel adjacently are the serial numbers, in the unique Siamese numbering system. If you remove the fore-arm of the rifle, you will find that the good folks at BSA were kind enough to stamp the serial number AGAIN, this time in our numbers, just below the Siamese serial number, but hidden by the woodwork.
And here is a point to consider. In the Viet-Nam War, which lasted 11 years in all, the Americans found that the barrel on a Colt M1911A1 was lasting a WEEK in that awful climate. Your old rifle was in that same climate for 40 years! It is nothing short of a miracle that ANYTHING survived of these very rare, poorly-known, underappreciated and undervalued rifles.
You have a very real treasure: a rifle purchased personally by a King. Treat her well, tell her you love her and feed her either .312" flatbase bullets or cast-bullet handloads and she will do her level best to let you know that she REALLY appreciates being out of South-East Asia!