I looked at one of mine (Oct 65, San Jose) and I've replaced the hardware but I remember that one (LH?) side had a hex head tapping screw. I thought it was a "later life" fix for a lost screw/nut combo. I replaced it with the screw/nut set-up. I would attribute this to two different containers of parts on the different sides of the assembly line. What I've also seen is the use of two different nuts, 378715-Sxx (finish was lost to time), 10-24 stamped hex, or 359606-Sxx, 10-24 formed hex (the former is the same as the four used on parking lamp retention, the latter on back-up light retention) which is the type shown in a picture that Dave has a few post up. I then (like about ten minutes ago) looked at 6S1232 (Feb 66 build) and it has the hex head tapping screw on the left hand side holding the socket in place (there is no light on the right, a work in progress). There is supposed to be a spacer (C4ZB-13A739-A) used between the socket and the dash for 65-66 but I can't remember seeing one. There are different part numbered screws, nuts and washer (as a spacer) documented for 1964.
Jim