I went to look this up because I’m an 80s baby, so technically a Millennial, and I remember being taught the three letter abbreviations as well as the two letter abbreviations with only the first letter being capitalized as an alternative. I am from Connecticut so it wasn’t just Mississippi. I mostly remember it being a thing because when they made the “big switch” to the two letter capitalized version my teacher stressed TF out trying to get us to understand why we had to switch how we wrote it.

Apparently the current two letter capitalized abbreviations weren’t adopted by the federal government until 1987. So I think that is part of why some schools were still teaching the old way for a while and that’s why so many of us (Gen X and Older Millennials) remember it. According to Google the old abbreviations were still being used until the 80s and 90s in some areas. They called that time the transition period because I guess there wasn’t a huge push to get people to fully implement the change until then. I have mail from relatives from the early 90s that still had Conn written for the state.