I have found one more photo of same gun at old version of "even more Thai vehicles" topic. Unfortunately many photos are lost now but I have a lot of stuff saved so I uploaded the photo in photobucket.
A field gun "in action" on the Shan front:
The gun is quite big and looks more like to field gun than to mountain gun...
An assumption. Maybe these guns were built in Thailand (for example as "Type 63 Field gun" that we can see in some lists of Thai WW2 weapon)?