Based on the components/parts list for each distributor, it looks like the Chrysler engineers specified a change in the “SHAFT, Governor and Cam Drive”, and also the “CONTROL, Vacuum”. These two items are the main features of a distributor that alter the rate of both the mechanical and vacuum advance curves. They must have felt a new curve for both types of advance were needed to improve the engine function/performance/emissions, etc., and these are potentially a few reasons why the change happened mid-year.

One of the ways I’ve identified early and late assembly line distributors on 1970 models is by the vacuum advance. Early units had a “long snout” adjustable advances, and later units had the more familiar “canister” style advances. The advance on the distributor pictured by AzMoparMike is the later style.