We all have great minds, because we were all thinking that. Well, at least the smart ones among us
HBT is not a common cloth for pure commercial items. Meaning, the shirt you pictured might be 100% commercial, but the cloth might very well be from a military production run (accepted or failed). The shirt design certainly has a civilian feel to me.
If I recall, a number of countries (Thailand included) went through a period of using hbt fabric on their uniforms. Phased out for obvious reasons. So at this point I'm not writing it off as commercial, but it is a strong possibility.
Yup, Asia was pretty big on HBT for a while. Lots and lots of stuff made in South Korea in the 1960s and 1970s. I'd definitely grab hold of this and hope for the best. My personal guess is the shirt is not military issue but the cloth either was or was intended to be.