Doing some research and came across this thread. Not to muddy the water.....
Took care of that with making it's own thread
but have a Dearborn built 7F01S135051. Original, unrestored with 35K miles, built 12/19/66.
Has the masonite filler board. What caught my eye on this thread is the difference in fasteners.
Scott is referring to the differences between the hardware used on this car and San Jose cars (where the post was originally posted)
Looked through my stuff and can only share the following
Only a small sampling of cars had this area exposed
No similar attaching hardware was found like what Scott showed in the first post - though, again very small sampling on my part
Figured we could include the masonite to cardboard change over in the discussion unless others thinks it would be better served separately. Just let me know and I'll change it
Changed over data found for the screw in (early 65-66 style to the later 68-up style) is as following
Latest example I have - 7F1275xx - masonite with screws
Earliest example I have - 7F1748xx - cardboard no hardware