Shanghai 7's series - model SS7
The finial standardlize model of 7's. It could be found both calender and non-calender models of SS7 sharing the same back-case. I would say this is the finial and the latest model of SHW factory made SS7's "Tongji". Since the quick-set date-just system of the preview models, 7221,7621... are so fragile, it turns factory to made a low-tech but more reliable canlandar seting system. Date adjusted by pulling out the watch stem in a halfway, keep turning the minutes hand forward to 12 o'clock and backward to 9 o'clock position to have the date-just.
SS7 case back are very common found in the maket.