Yeah, I noticed this like over 20 years ago.. From what I could gather, The square style was used in 1965 -Probably just a different supplier.. I don't ever remember recalling seeing the square style in a 1966 car.. I think the round was used for bothe years 65/66.
Wow, I thought everyone knew about this one..
It sounds like a good poll is in order for this..
Tony K.
Tony, I'm like you and have known this for a long time as well.
It's a different supplier and they also won't interchange, the combination
of wheels is a different length. My Oct 64 fastback (Falcon style cluster)
has the rounder style numbers. It seems to be pretty random on
which type you will see on 65/66 Mustangs. In looking in salvage
yards at 60's Fords the early 60's cars all look to have the square
type numbers and in 65/66 both are used then starting in 67 they
all seem to be the round style.