In the 1970s they had “G”, “A” and “AO”.
“C” was created in 1979.
In 1981, “A” became “PGR”
“P” for pre-school shows was introduced late 1980s sometime, maybe 1988?
In 1993, “PGR” became “PG”, “AO” became “M”. I think to align the classifications closer to movies.
“MA” and “MAV” came along either at this time, or later. I can’t recall.