The clip pictured above, in my Reply # 32, is from National Parts Depot. It is actually noted for a 1957 Thunderbird ; entering the Part Number (358350-S) in their "Search" in the Thunderbird Catalog will bring it up (of course now, if you go to their Site, it shows "Out of Stock" at all of their locations guess I cleaned them out).
As far as the "bubble" on the coating, it's not just a "bubble", but many small "bubbles" (zoom in to see them). Over the years, I have seen them do this not only on Mustangs, but on my '57's and '59's also. I truly believe that the "bubbling" effect is caused from engine heat acting upon an "ultrathin" layer of rubber coating. Obviously not hot enough to affect thicker rubber components, but enough to affect the thin layer on clips.
Bob