Here's a picture of four types I "acquired" many years back from a local car "dismantler" who specialized in Mustangs. As you can see, there are many types. The one on the left I do not believe is a Mustang bracket, but who knows. The next on it the longer version used in early 1964 Mustangs. The next two were the type used in 65 and on Mustangs. Note the one on the right is just a bit shorter than the one next to it. As to finish, they appear to have been "plated" or at least treated. Most of the one I got were rusty, pitted and had some type of corrosion that looked like they had salt water crystal specs - my interpretation.
The "1965 Mustang Body Assembly Manual" (the infamous MPC) - AM0001, page 36, lists these brackets as C4ZB-6513190, as does the 65 Body (AM0006 pg43) and 66 Body (AM0011 pg39). They are not shown in 1967 (and up, I assume - not in the 68 Body or Interior). I tried to find them in the 75 edition of "Ford Car Parts" but could not find them. It is quite possible that Ford did NOT service this part.
Jim