That spring nut (p/n 380264-S2) was used on the right hand Fastback trunk torsions rod from 65 into the 67 model year when the 68 style hinges were supposed to be implimented. I found a spring nut on a 67 San Jose, May, 67 build Fastback so I don't know if or when it got done. I got a damaged spring nut from a local Mustang shop and managed to clean it up enough to read the manufacturer's name, PAL. Palnut and Timmerman (the other major supplier of spring nuts) are now one in the same company but it makes no difference, they don't reproduce the part. However, they have a similar item, same OD, same ID, same thickness, but it does not have the 8 retaining serrations that the original has, it uses a formed lip to retain it on a shaft. I got some samples from Tinnerman (it pays to be straight up when talking to any salesmen and let them know what you are trying to do, this one had a 69 Mach I) and used a Dremel tool with a diamond cutter to "add" the serrations. As the spring nut is painted, you cannot tell. After I put one on my 66 Fastback, a 67 Fastback and two 66 Shelbys, I got about 10 left. Send me a P/M and we'll talk.
Jim