Per the Mustang Assembly manuals, 1964 and 1965 used 357036-S100 (AM0002 pg 7, AM0007 pg 7) which was a single pin type, and 1966 and 1967 used 377347-S2 (AM0012 pg 6, AM0017 pg 6) which was a multiple (12 ?) pin type. Pictures of both types with dimensions are shown in the AMK Guide To Ford Fasteners, pg 237. I strongly urge those making inquiries into hardware to get a copy of this book as well as getting a set of the Mustang Assembly Manuals for their car's year.
As to differences in types (are there more?), 66 San Jose built Mustangs used 377347-S2 based on cars I've worked on (Oct 65, Dec 65, Feb 66, Mar 66, Jun 66 (I think it was June)).
Jim