You may recall that passenger car and certain other plates in Indiana began expiring on a staggered basis with the 1970-71 issue. However, the new registration system wasn't applied to trucks until much later. That's why things get a little interesting beginning with the '70 issue. It actually expired on Feb. 28, 1971, and so on.
Color schemes continued to match each year's passenger plate until 1977, when passenger cars got the first in a long line of new graphic designs.
The embossed corner boxes introduced in 1967 continued in that form until 1974, when they were squared off.
You may also notice that the state abbreviation and year traded places with the weight class indicator until 1975. Beginning in 1976, the abbreviation and year moved to the top and remained there for the rest of the decade and beyond.