Well you have ruled out the radio delete idea that first springs to mind and some others, in the past, have guessed was the connection.
Looking at other 67 NJ cars I found others with the -R coding with factory radios according to their Marti reports/Ford information. So we have multiple confirmations that its not a "fit"
A simple way to look at much of the coding on a buck tag is that this was the way workers knew what holes and brackets needed to be present before painting so that they were present when it came time to install parts on the car. Looking at a number of studies that have taken place over the decades that included or focused on New Jersey plant. Also the codes were different depending on what plant and year - even the time of year according to workers so be careful of studies that don't separate or ones that borrow codes from other years and plants
So looking at my collection of buck tags and cross referencing those with pictures of the car as well as Marti reports and/or buildsheets if available I think I found the link.
The "-R" appears to relate to the lack of rocker panel molding. Since rocker molding came on allot of 67 Mustangs and there were two rocker panels produced during part of the 67 production year (one of the running changes) this code told workers not to drill holes in early production and in later months which rocker panel (with out holes) should be installed on these specific cars.
Open to discussion or other options but so far have a number of cars with confirmation that helped me form my conclusion at this present time. Will offer I do have couple of examples with the -R with rocker panel molding but those were likely added by a later owner or during "restoration". On at least one of these the buildsheet shows no rocker molding originally.
There were other examples with buck tags without "-R" in the search that had rocker panel molding to confirm the buck tags stamped that way to help confirm the finding suggested
Hope this helps
