There are two differences, but only one is the caliper itself (the other is the retainer, one large retainer on early models and two smaller ones on later). The major difference is the dust seal and how it attaches. Early calipers have a steel ring that the seal goes around (the seal can be installed after the piston) while the later has another groove cut into the cylinder. This is where rebuild kits are critical. Some kits have four of each style while some only have the early seal. This is all because that seal is the same as the 65-67 Mustang.