Since I have a C7OA hose attached to these 65-66 style Kelsey Hayes calipers, I'm inclined to think that there were at least three different styles used in Mustangs:
1. 65-66 AND early 67 with 3/8-24 NS hose fitting size, external boot retaining rings, and single full size brake pad retainer.
2. 67 with 3/8-24 NS fitting size, internal boot retaining rings, and dual small brake pad retainers.
3. late 67 with 7/16-xx fitting size, internal boot retaining groove, and dual small brake pad retainers.
As Bob mentioned hardware differences are also in play, but I'm concentrating on the big picture at the moment.
Looking for comments and corrections, especially in the 65-66 and late 67 hose fitting sizes.
Bob also stated in the related post that "Early build SJ 67 car's used the small inlet like the 65/66"
So it could be that thr 65-66 and early 67 are 3/8-24 NS, and the late 67 are larger. If that is the case, then the early 67's may be a direct carry over from 65-66.
As for trying to determine the change over date(s), I can offer:
1. 7F01C129xxx Dec 3 scheduled build, has the 65-66 and early 67 caliper style.
2. My156855xxx Dec 20 actual build car had probably more than one caliper exchanges by the time I got it in 1979
3. Jeff's posting above of "original example from a December 15th scheduled car" that is the early 67 style.
4. Richard's (67gtasanjose) 11/02/66 had the early 67 type http://www.concoursmustang.com/forum/index.php?topic=8827.msg51459#msg51459
5. Jeff's other examples from March, San Jose, of the late 67 type.
To confuse the issue, I have some 3/8-24 (early) threaded calipers with the late style grooved boot retaining rings, although they appear to be aftermarket, with no K/H markings. Will post pictures later.