The Ford part number for the spring nut is 380264-S2 and is shown in the "67 Weld and Sealant Assembly Manual" on page W7-8345. Toward the end of 67 production, a design change eliminated the spring nut but I don't know the change date.
The same spring nut was used from early 65 thru 66 production. I mention those years for sources as the spring nut is not reproduced.
Jim