I'm 174 with 80 inseam and ride a S/M size (the L is 2cm longer on top tube). I dialed mine (early Smooth hound = frame does not fold) like my gravel bike but a bit more aggressive (handle bar is 15mm lower).
last year, I rode the frame as a fixed MTB/flat bar gravel with a shorter stem (90mm), taller steerer, dropper post etc. and now I ride with a shorter steerer (40mm shorter, not shown on picture as I got raid on the big black spacer and 1 purple spacer), a 100mm stem with ritchey cyclocross handle bars (I had 110 stem and road short reach handlebars) and a carbon seatpost with 25mm offset.
in both cases, I ride with 165 cranks. I built a hammerhead 7 (suspension fork) with 170 cranks and I would not go longer as it would hit the ground in every corner.





Point I am making is that if you really want one of these bikes, get a frameset with a full length/uncut steerer. The difference between the 2 frame is not that big and you are at the limit between the 2 old frame sizes. Note that on the Japanese site, they don't seem to have 2 frameset size anymore, they use different stems to differentiate Size M and Size L.

These framesets are using very common standards size so you can fit whatever you need to suit your morphology and fitness.

Many thanks for your helpful info and suggestion.
I'm sceptic on size M Altena as Dahon specifies that it is for 157 - 172 cm. rider height while size L is for 170 -195 cm.'s. However if size L's cannot be got hold of, I might weigh and compromise on sizing keeping your suggestion in view.
Btw for my sizing comparison info, could you give the top tube length (c-c) and wheel base of your Smooth Hound (presumed size M) ?

Abu got his Altena in Singapore rather than in Thailand.