Like it was said above; I have absolutley nothing to scientifically base this on other than the years ive spent playing my 4 short scale guitars. Here are my findings:
* '69 RI neck is more of a "C" shape when compared to the Jag-Stang
* Jag-Stang has more of a "D" shape when compared to the '69 RI
* '66 B width Musicmaster II neck is more substantial in the hand (but definatly not Baseball Bat) than the '69 RI
* '64 A width Mustang neck is slightly thinner/or maybe faster than the '69 RI. My personal favorite neck (but not guitar!)
So if i ordered them from thickest to thinest i would be:
B width -> Jag-Stang -> '69 RI -> A width
PS I have never owned a '65 RI & have only played them a couple times.