Per the 66 Mustang Assembly Manual, AM0011 pg33, I think you want item "J", 378695-S2 5/16-18X1/2hex with an external lock washer. These look like 64-65 fender bolts without the lead point. Look at the attached and see if these are the ones.
Just because you got a certain type of item on one side that is supposed to be "original" and a different one on the other, do not assume that the "original" is correct.
I strongly recomend you get a set of 66 Mustang assembly manuals, plus the 64 Chassis and 64-65 Weld and Sealant.
Jim