I never openned this 21 jewels Shuangcheng (double towns) watch before, and thought its automatic movement was the same as my Guangzhou 21 jewels automatic. But it is not exactly the same. It is still based on a 21600 bph standard movement, but the automatic mechanism is quite different. The manual winding gears are not engaged when not winding, and the layout of other gears are not the same. The other movement is a 33 jewels Beijing railway automatic.
This watch was made by Beijing Watch factory probably in late 1980's. On the dial, it says 17 jewels?! :-D and it is a date automatic watch. I will post an image of this watch later.
So this is a quite uncommon automatic standard movement.