The date code BC is B/second year of production and C/month.
The SS1A was a major development/improvement that followed an earlier set of movements much more closely based on the first Shanghai movement, the A-581. The first SS1A movements were produced in 1966.
Although arguably still based on the A-581 series, I see it more as a transitional movement between the more typical 581 movements (the others in that series all looked quite the same with only minor visual differences) and the Tongji or standard movement.
Here's a 581 movement for comparison.
...and a tongji/Chinese standard movement.
Curator: Alberta Museum of Chinese Horology in Peace River; Member: China Horologe Association; Chief Expert: China Watch Overseas Working Committee (CWOWC); Sino-Canadian Horological Cultural Affairs...