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It’s a pretty obscure piece of trivia, but there was a knob on the idle mixture screw around the 1970 timeframe. I posted a picture of an NOS one once. I’m not even sure where it is at this point, but I’m sure I still have it…somewhere (just like @ToyotaMatt decalsIve seen the distributor with the knob but never a carb.

But that wouldn’t have applied to the vehicle year he mentioned to you, so it’s more likely that he was referring to the octane selector.