Hmmmmm.....this is a strange one.
Your "local expert" could well be right. Your rifle could be a Siamese Contract Type 38 with the forestock and handguard missing.
That's certainly a "Charkra", and the writing appears to be Siamese.
There's a picture of a Siamese Contract Arisaka Type 38 here:
.....there are differences between your rifle and the one pictured...
1) Straight vs. bent bolt
2) Bolt "carve out" in stock vs. none
3) Sling "carve out" in buttstock vs. none
4) Distance from front sight to handguard
The most puzzling thing is that your stock appears to be original, and the bent bolt also appears to be original, and differ considerably from the Siamese Type 38 in the link above.
It could be
that there were more than one type of Type 38 supplied to Siam by Arisaka under various contracts.....could well be that the Type 38s supplied to the Siamese Police differed from the ones supplied to the Siamese Military. That's the only explanation I have for the differences between yours and the one pictured in the link above.
Here are a few good Arisaka websites (but they don't show your rifle):
Sooooo.....my best guess is that your "local expert" knows what he's talking about......that you have a (quite rare) Siamese Police Type 38 Arisaka Carbine.
Sorry I couldn't nail this one down for you, but hope this is better than a lump of coal in your stocking.....