Just checked my information on NP heralds and slogans, and what I have is this regarding dates.
First use of "Mainstreet" slogan;
Brainerd built cars 25000-25999 in October of 1947.
4/4 Ends, SQ panel roof, Superior doors.
First use of 60" Monad;
Brainerd built cars 24000-24499 in November of 1951.
4/3/1 Ends, Diagonal panel roof, built with straight sill length of car.
If anyone had additional information, please share.