If your asking about the pinch weld (the spotwelded edge used to join the rocker and the inner rocker) then yes it was the practice of all plants to black this out, before the front fenders were installed from the front wheel well to the rear valance. It is common belief that this was done on all Mustangs other than black cars. Some others believe that some very dark colors were skipped over also but in recent years some original paint examples have suggested that even these got the black out but currently very dark colors without are not loosing points for not having the detail.
This is a very often over looked detail than many home restores seem to miss though it only takes about $3-5 to correct and about 15 minutes.
Normally the black extends on the bottom horizontal surface of the rocker, out from the pinch weld by approx 1/2" on average. The edge was not taped off , so we would expect no sharp paint edge but instead a soft paint edge as would be produced by free handing the application with a low pressure gun at about 30 degree angle or with a paint shield. Most painters use a back masking technique to reproduce the soft edge and in turn protect the upper surfaces from overspray
Hope this helps
From a 67 but some thing - REMEMBER the black overspray on panels and edges that hung down below the car. Also consider there was more overspray than shown in these pictures originally , lighter spray likely was worn away of the years.
