Nice work investigating Uke. I'll add my opinion that the thumb rest variation is independent of the changes between 65 and 66 that you noted. I say they used what they had when they had it, experimented at will and sold anything that worked/functioned without flaw.I've noticed many differences in my 1965 and 1966 Jetstar 6-string guitars. Nut width, contours on the body, thickness of body, species of wood, neck thickness, fretboard radius...the headstock and tuners on those guitars differ as well (same as the basses from 65-66). Overall they are completely different beasts.