Vacuum Advance

What would be the correct vacuum advance unit for my 68 390 S code with an automatic trans. Single or dual advance?

My '68 shop manual shows dual with thermactor(without A/C) but doesn’t say if with A/C.

Your car could have come with either. The stock distributor early in the model year was C70F-12127-F with single advance. Late in the model year there was a TSB showing that some engines might have C8)F 121227-D with dual vacuum advance.

Royce, my car is a Hertz car build date 3/1/68. Should have put that in my first post.

It will need the more expensive and hard to find C70F 12127-F in that case.

Thanks Royce…