To attach the arm to the grill bracket, just a nut is used, 55738-S36 (3/8-16 NUT, 0.363T, FLNG 0.793, 9/16 HEX, SERR). It's the same nut, except for the gold zinc finish, as the ones used in 1964-66 Mustangs to attach the shock tower braces to the firewall at the firewall. It has "teeth", or serrations on the mounting surface. The bolt, 380070-S(
(3/8-16X7/8 SPC SQ HD 9/16, 0.067T), is a special, thin, square head bolt from the arm thru the grill mounted bracket. A flat nut spring nut, 357890-S(2) (3/8 NUT, FLAT, 0.630L, 0.500W, ?T, SPRING) was used to keep the bolt in place during installation. Remember, the fog lamp grill is different than the standard grill for 1965.
To attach the lamp housing to the arm requires a special cupped washer, C5ZZ-15251-A, and a nut, 373403-S8 (3/8-16 NUT, 0.344T, FLNG 0.750, 9/16 HEX, LKG). The same fog lamp housing to bar setup is used in 1966. A quick scan of a couple of current Mustang catalogs does not list those items available as separate.
That's the description, I got to reconfigure my computer's software to work with the new printer's scan feature. Part of the installation is shown on pages 40 and 42 of the 65 Mustang Electrical Assembly Manual, AM0008. The assembly of the lamp housing to the arm is shown on page 1 of Ford Car Parts Illustrated section 130.3.
Jim