Not sure what you mean about the two pages agreeing. If its the color names it might just be that they took their information from the same source
See the attachments below. These are for 1966, but as far as I know the paint companies didn't change formulations for a given code number. For Ford paint code "4" (Silver Frost - Mercury called it Sheffield Silver that year), PPG lists 32520, and DuPont lists 4733-L. MA therefore lists them both as matches for Silver Frost, along with Ford code M1901. Not sure where they came up with the Ford code, it would be interesting to follow that trail. So, PPG and DuPont both list their "Silver Frost" numbers as the silver stripe color for 1971. They agree on that point, but of course that doesn't conclusively prove that this is the correct Mach I stripe color. They could
both be incorrect. Or they could both be referring to something other than the hood and rocker panel treatment.
FYI both codes come up with the same matches in the carpaintonline.com database. ("4" for 1966, along with "M1901A" and several other codes through the 60s, 70s and into 1980)
Interesting that both ignored (just based on these two pages) the whole Mustang/Mach I hood and body treatment
DuPont and PPG refer to stripes. Is the hood paint a "stripe" to them? Is the rocker panel paint a "stripe"? Perhaps - not the term most people would use though. Rinshed-Mason uses "stripe" but they also use "exterior trim colors" (with a different code than their "stripe" color). So maybe they did ignore it, or maybe they just didn't worry about semantics.
I think the best next step would be for Ashley to get one of these color chips from ebay or a swap meet, and see how they compare to a car with original silver hood/rocker paint. Or get a fresh sample of one of the two colors from carpaintonline.com. Probably not a very easy task to find an original-paint vehicle to compare to, and paint on a vehicle and paint chips can change over time too. The only other alternative I can think of at the moment is to contact Ford and/or the paint companies to see if they have any additional information in their databases or archives.
For the record, I'm not claiming that any of these codes are a definitive match. My intent is to provide the data I've found and point out consistencies and inconsistencies that will hopefully provide leads for further research.