After some searching, found this page indicating that "MI" stood for "Master Item" number.
http://uv201.com/Microphone_Pages/rca_junior.htm
Edit:
Here's an RCA spec sheet that spells out the term:
http://www.oswaldsmillaudio.com/imag...07/MI12484.pdf
2nd edit: and an RCA broadcast equipment catalog in which everything is listed by MI number:
http://www.davidgleason.com/Archive%...log%201962.pdf