The way I read the chart is that 1982 started with 75-97690 as the 1st rifle shipped that year, the 76- prefix was skipped. !st rifle shipped in 1983 was 77-50834.
Your rifle was most likely made and shipped in late 1982. The 1st rifle of 1984 was 77-68636. Don't know where you picked out the 77-9xxxxx- which BTW is one "x" too many.