If a certain plant or factory had an overstock of parts , or not enough , why not
Keep the line running

No different then production 71 Charger s with the concealed headlamp option having a mix of the 1971 and 1972 front parts / headlamp doors or 1972 concealed headlamp parts all together

I don't think there is an exact timeline ? More or less what was available at an individual factory