The SCSW oversimplifies several of the K serial ranges. The 9K numbers were used over a period of three years, 1976-78, as follows:
1976: 9K1 to 9K13999
1977: 9K14000 to 9K83999
1978: 9K84000 to 9K99999
Hence, 9K887xx would have been serialized in 1978. That, of course, does not mean your gun actually shipped that year. It could have shipped later.