In February 14, 1966, Ford added an addition clutch return spring for Mustangs with manual transmissions. I assume this includes Shelbys built after that date. The spring used a retainer that came thru the firewall using a vertical slot in the firewall. The retainer was held in place with "tangs" (see picture 4, -13). The flat piece is a seal, material unknown.
Here's what I know.
Pic 1 From the 75 Ford Car Parts, ILL75P05.
Pic 2 thru 9 (-11 thru -18, sample 1)
Pic 10 thru 12 (-21 thru - 23, sample 2)
Pic 13 a sketch
Note: The original part appears to be natural steel, possibly phosphated or painted black.
Jim