Estonia/Latvia/Lithuania were recipients of VZ 24 rifles chambered for the 303 British I believe. Supposedly they each received 15,000 of these rifles shortly after WWI. Personally, I've been searching for one of those for close to 50 years and most collectors seem to think they are only rumors. But two different authors claim to have seen them and state they exist.
There were also some custom rifles made up on 98 actions chambered in 303British. I believe Rigby and Holland & Holland both made them on special order.
What I wonder about is that your rifle has a Fajen stock. I sincerely doubt Fajen would make a special run just for a very limited amount of these rifles, which to my knowledge never entered North America in any quantity. Mind you it wouldn't be difficult to modify a Fajen after market stock made for the Siamese Mausers.
I have seen Siamese Mausers converted to all sorts of different rimmed cartridges, including the 303 British.