Abbreviations:

I remember when I was in school having teachers teach us the traditional abbreviation of our state, "Mich.", and then teach us the new fangled two digit "MI". They made a point that the government wanted everyone to use the two digit way now. That would likely have been in about 1980 or so.

The two digit system for state codes went into effect at the post office in 1963 and for the rest of government in 1987. The 1963 innovation accompanied the zip code institution.

Just something weird I was remembering.