Hello vonofterdingen, I made two campaigns many years ago for the Thai/French conflict, the thai version used a 'th' folder in the campaign folder but I cannot remember why it used lower case initials, it might not matter if you use either upper or lower case.
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The French campaign used the 'vi' folder for Vichy, again in lower case.
Thai units also fought alongside the Japanese occupation forces, there are debates about whether they actually invaded Thailand or not, they did as far as I am concerned, so in that instance you could use a Japanese folder for your missions/campaigns.
The old 'Forgotten Countries' mod actually had Thailand as Blue and Red, the insignia were different, standard roundels or the Elephant insignia, however, sorry to say I cannot recall what folders it used.
My own B.A.T install has 'TH' in the single folder and 'th' in the campaign folder, nothing more, however the countries mod act notepad readme gives 'TY > Thailand (Postwar)' but I have no folders for it but assume it is built into B.A.T.
Hope that helps.
Take care and be safe.
Wishing you all the very best, Pete.
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