It does make life a little easier having these models not running concurrently. I imagine that Valjoux ceased production of the movements forcing Heuer to update their models. The reference I'm using is entitled 'Chronograph Identification Manual 1970 - 1971'. I would guess that it was written in 1970 at a point when the 1971 models were known. Unfortunately there's no dating information for the changeovers, I wonder if knowing when Valjoux phased out their movements would give us some idea?

Bruce