The model debuted in 1978, and the "B" series ran until 1979/80. AP rolls to a new letter every 100,000 units, so how high is the numeric figure after the "B" on your caseback? That's your key to whether it was made closer to 78 or 80. I want to say it was discontinued in the early 1990s, but I'm less certain of this fact. If Thomas is around, he's likely to know the close of production with greater certainty.