What about my assumption that these guns could be built in Thailand? Look at the Thai "Historical equipment" list in Wiki. The only gun with Thai origin is some "Type 63 Field gun"!
To get the western date of model we should subtract 43 from thai date (63-43=20) so we have M1920.
We well know that Thailand acquired many Bofors guns of different models but we don't see them in Thai military museums (I saw only 40mm AA gun). I'm wondering why just this strange gun is presented until now? Why we don't see more modern and powerful Bofors 75mm field guns (75mm L/40 M1934 - 52) or 105mm and 150mm howitzers on military parade video? These guns would look more showy for people! IMHO: because "our gun" is dear to them! It indirectly confirms the version of Thai origin...
What do you think?