I read in the FSM that they began the marks in '82 but didn't start using them until later in the model year. I think they said August.

The upshot being that you should self-index the '82 shafts whether they have the marks or not. Anything after that just use the marks.

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1981 300TD 310k miles
1970 280sel 172k miles
1966 230 Fintail 162k miles

"Where are we going? And why am I in this hand basket?"