You find on this page the factory black out area for all 1970 Mach1 except for Mach1 with black exterior color. A common restoration mistake with 1970 Mustang Mach1 is to paint the floor side outer member assembly (or pinch weld seam ) and rear quarter panel lower flanges body color, when they should be blacked out.

The idea in the factory was to have these seams, which were visible from the side of the car, blend into the undercarriage.
The white car is a unrestored Mach1 and the other green Mach1 is a restored car ( Note the same black out located between door and rear quarter panel ).
The calypso coral Mach1 & white Mach1 with out the black out area  on the pinch weld  and between lower molding.
This Mach1 with correct black out area
Note the exceed of black paint inside the lower section of door
This original Brown special order paint on San Jose Mach1 with factory black out paint
The correct black out area on the rear back panel on Mach1.
The rear back panel with black out area.
The rear back panel and lower rear quarter with out black out paint.
The rear lower quarter panel & pinch weld with black out paint
Note the black paint around the trunk lock
The pinch weld with black out paint. ( note the overspray on the torquebox for factory appearance ).