Dear khi,
My guess is, as proposed by Tony, this cover was posted at Dongchuan. The Dongchuan cancel could be read as "DongChuan / 37 July 21 17 / ChungKing (Area)(?) ". The "37" is "the 37th year in Minguo calendar" and is 1948; July 21st quite obvious, and I suspect that "17" means "17 o'clock". The roman datestamp of Chungking says "21 7.48 | 24", which suggests that the receipt time was "24 o'clock", July 21st. So Dongchuan cancel applied before the Chungking, thus Dongchuan the starting point.
A conjecture on Dongchuan. The bottom inscription of the Dongchuan datestamp indeed says "ChungKing" with a rather blur small character which might be "area", and this may suggest that DongChuan (TongChuan) is indeed a place in the vicinity of Chungking, and not the Dongchuan/Tongchuan near Kunming, Yunnan which is quite far away (a couple of hundred kilometers) from Chungking. It is hard to say but it seems to be quite unrealistic that the postal system at those days was so well organized such that a posted letter be transferred by couple of hundred km during the night... To confirm this conjecture we need to check the detailed post office location at Chungking at that time, so any comments from China experts are welcome.
Another interesting discovery; the "27" of Montreal P.O. is inverted :-).

Cheers, unechan @ Osaka, Japan