On the 1965 Mustang, the lower clutch release rod (clutch equalizer bar to the clutch lever at bellhousing) was nonadjustable. This was due to the fact that the top rod that runs from the equalizer bar to the pedal was adjustable. This nonadjustable rod was made obsolete from Ford quite a while back, so lots of folks just used the '66 adjustable one.
The 1966 Mustang had an adjustable lower rod because the upper rod to the pedal was nonadjustable.
Got all of that?
Anyway, the 1965 style nonadjustable one is available again. The adjustable '66 style is still available, of course, also.
Here are the part numbers: