I found the drawing in the 1967 body assembly manual, but not the 1966 version. For 1967 a 380020-S2 is called out. That particular "J" nut is not listed in the Ford fastener catalogs that I have access to (1959/60/61/65/69/75/2004). I don't have any AMK books. A general Google search did not yield any results. Jim of course has this same information in the 1967 version of the hardware spreadsheet.
Then I checked the 1966 hardware spreadsheet. It is listed here as well, but the notes say "NO 66 DWG" - which is exactly what I found.
After that I went back to the 1965 body assembly manual and found the info - the bottom line is that four 380020-S2 is what you are looking for. As far as the exact manufacturer for the assembly plant and time span, that of course can't be provided by any documentation that I'm aware of.
I have the "extra" advantage of having several (I'm down to two) 1966 Mustangs, all with 289 V8 engines, to use as a reference, plus I carry a clip board at car shows to check out other Mustangs, so when I enter any hardware item into the master file (as or today, it has 10,533 lines of hardware data) - I am absolutely sure of its accuracy (mostly).
I have not looked into the part number of hardware. What I'm looking for are for a t-bred project and these could be plant specific as well.
And there lies the problem. You have no idea what you are looking for. If you want accurate "thoroughbred" level hardware, you need to know the Ford hardware part number as used on the assembly line. Those part numbers are documented in the Jim Osborn Productions, Mustang Assembly Manuals. To obtain "pristine" replacement items, be prepared to buy a lot of "parts cars". The other option is to buy new. Your choice.
Jim