Originally Posted by
CV-6
I was sold a Suntour with the "M" on it and it was not French, so not a 100% thing. I have also seen the "M" on a Suntour on the backside of the body. That one was French, but I have no others for further support. Best bet is to find a thread gauge to check. Thread gauges are cheap at your local hardware store.
FWIW, I have a SunTour freewheel that is explicitly marked as metric thread: