[When did the M100/Bx20 split occur?]
It was in March of 1989.
The old M100 was W-A-Y too long for its own good. Buses kept getting lost at the south end, so dispatchers would turn some short, leaving Riverdale with lousy service.
The solution was to divide the M100 into two routes. The portion below 207th Street remained the M100. The portion above 207th Street became the Bx20.
In addition, the Bx7 was extended south in order to adequately serve the Riverdale-to-Washington-Heights market.
Interestingly enough, the major complaint from Riverdale riders was NOT about the routings. Rather, they were upset because their primary bus service no longer had a Manhattan number!!! They didn't like being reminded that that they were part of the Bronx - the 718 area code and the 104xx zip codes notwithstanding.