There have been forum threads about this in the past with copies of adverts from dealers in the US dating from the late 1960s / early 1970s.
It's certain that several container-loads of M20s were taken over and most bear Belgian customs seals suggesting that they came out of the BAOR REME depots near Antwerp.
The most surprising thing is that so many of them have survived in pretty much untouched 1960s 'demob' condition. My suspicion is that these rigid machines were seen as donor 'hardtails' in the wake of the 'Easy-Rider' craze but that many of the owners were dreamers who never got round to it...
I can't imagine that military-looking WM20s would have been a popular look amongst the alternative motorcycling culture of Vietnam era America...but I only know the U.S. from films and television so I could be wrong 