I’m working on a friends ‘70 Mach 1, most of the buck tag makes sense but there’s a code of Platinum that has me confused, could it be for the stripe on the seat, it is silver on this car
Pretty typical for 70 NJ. That would be the name of the exterior color "called out" It appears NJ dropped this practice (code with a paint color name) later in the production year.
These codes and names will not always match what you will see in a decoder. Names for each code may have been from another line at the same plant rather than Mustang just to make things easier on the workers.
Do have at least one other 70 NJ with the "N Platinum" shown
On the second question - The buck tag didn't have anything to do with the details of the interior other than listing the body type and an option that required a different hole, bracket or some other item before paint and for paint codes for the painter. Reason why the codes with there. On some years and plants these were rewritten on the body itself so they didn't have to look at the smaller type on buck tags or buildsheets attached to the body