I'm still laughing. Through 14 replies did anybody mention the plug's part number, so I will - per Osborn "1966 Mustang Body Assembly Manual", pg12 "377949-S". It's dimensions are found in Osborn "Standard and Utility Parts Catalog, July 66" on pg 58 (my measurements are 0.750D HOLE, 0.100PNL, 1.312OD, 23/64T, BLK RBR). The most common use for 64-66 is the two hole plugs in the trunk over the frame rails. The plug is a common item used on all years to cover control holes.
Jim