Have to agree with Tom there. This is the clutch pedal assist spring ACP Clutch Pedal Return Spring Manual Transmission 1965-1968Oh I figured it out. It’s the under dash assist spring. Thanks guys
If, by early you mean April of 66 You are correct. That is my build date and my car has the little tab and spring.The early cars had a clutch return spring in the engine compartment. It was phased out 8n late 65 or 66. It connects to a tab on the firewall and t9 the to of the Z-Bar
I believe it is the assist spring. I already have the retracting spring assembled.1965 Mustangs with the adjustable Pedal-To-Equalizer Rod and fixed length Release Rod did not have the upper spring on the clutch linkage. Look at the Retracting Spring that attaches to a hole in the end of the Release Lever. I think the spring in the OP's photo is the Retracting Spring.
Where in the firewall?It is too light to be the under-dash assist spring by a country mile. No way, no how.
If the bent end (lower in the photo) were re-formed properly, it would be appropriate for the under-hood return spring between the upper equalizer arm and the firewall.