TFoley is correct: the numbers are Siamese/Thai, and are likely a registration number. The arm is a pretty typical 'monkey gun', used by the various hilltribes for taking game, though this specimen appears to be somewhat nicer than average. The hill markets stocked seamless tubing of about 1/2" bore, which local 'gunsmiths' used to make-up such arms, which are smoothbore. The markets also stocked saltpeter and sulphur, for making powder, though the locals burned their own charcoal. The arms are percussion, though the 'nipple' is commonly of large diameter and flat on top. Fairly coarse shot was also sold, though the shooter could make his own. Local apothecaries made-up percussion compounds which were applied in dabs to sheets of paper, which were torn off and stuffed into the large recess in the hammer nose.
mhb - MIke