As mentioned red oxide was the color of the epoxy primer used at NJ that year. Not the flat primer but a smoother primer sealer product that was epoxy based. Pinch weld black out has been found on allot of dark color cars from this period as we continue to see more original cars. Front frame rails ( on the bottom) and brackets facing downward in front of the firewall may be a slightly different tone of red oxide since that area was painted at the different station and time from the firewall rearward. Some times there was allot of body color overspray other times less. May be the results of an older or younger, taller or shorter painter as well as one putting more effort or less.
Here are a couple of pictures of sections of floors collected from 70 NJ cars
In this example not much of the body color or pinch weld made it through the clean up process. The blue arrow shows a section with the pinch weld black out remaining. Between the red and yellow arrows you can see some body color overspray as it would have "collected/settled" on one side of the floor rib facing outward towards the rocker. Direction towards where the black pinch weld paint remains.
Another example from just before your car was built
Here is a shot from a 70 NJ Mach. Since the black under the car is often light overspray (unless the jet was close to the panel) if often did not hold up as well as the body color over the years. Possibly the reason the frame rail in the picture shows little or no black remaining
In addition to the body color and pinch weld black out on the bottom of the floors your car would have also received a couple of passes with a wand applying sound deadener - Mach I, Grande and some Boss 302 thing
A few related (not exactly but in general) threads. Though they are Dearborn related
http://www.concoursmustang.com/forum/index.php?PHPSESSID=kfbltiebpa8f14ef2hnuvmran2&topic=7910.msg45383#msg45383http://www.concoursmustang.com/forum/index.php?PHPSESSID=kfbltiebpa8f14ef2hnuvmran2&topic=4217.msg23241#msg23241NJ cars due to rust issues are often much more difficult to get good pictures of - though Boss's since they were shipped further from the area tend to offer up the best examples in general
Shot of a 70 NJ with sound deadener followed another picture with the sound deadener area highlighted
In this picture you can make out the red oxide (to the left) where the sound deadener has been cleaned away, while on the right side still some remaining body color showing through where the sound deadener has fallen off
Extra picture showing how on this car even the rear frame rail got a decent coat of body color. And BTW while your doing all this work - don't forget the dolly marks
Don't want to be trying to do them afterwards - in reverse
Hope this helps
