I founded this little gem: http://www.nycsubway.org/bus/busroutes.html
Yeah, it's an interesting read.
What was the M105?
Looks like it ran on 10th Avenue. It was probably merged into the M11 when the avenues became one-way. (That's just my guess.) I remember the old M103 (59th Street crosstown, then up West End to 72nd) and M106 (42nd Street crosstown) but the M105 was dead before I was born.
Was there really an M58? It's listed under CURRENT ROUTE and seems to be like the M31 (which is an old number, and it used to go only to Lexington).
The M58 is pretty recent. The 57th Street crosstown used to be the M28. One day about 15 years ago, the routes around there were all changed. The M103 was cancelled and the new M57 took over its West End run. The M30 was cut back to 57th and 8th (it used to continue up Broadway to 72nd and back east to CPW), as the brand new M72 was more useful as a 72nd Street crosstown. The M58 ran down York from 72nd to 57th and then across 57th to 8th. The M31 was untouched; it continued to make its wide loop, terminating near Bloomingdale's. I had a brochure describing all of these changes but I'm afraid it was thrown out a long time ago. (I'm not sure about this, but I think the M29 became the M66 the same day.)
A few years later, I guess someone noticed that the M58 and M31 had a long overlap, and they were merged into the M31.
When were the busses given an M prefixed and renumbered to prevent conflicts between former companies?
I have no idea. Before I was born (1974), that's for sure.
What was the old M12 (shown as 86th-York, not Spring-Delancey which it was more recently)?
Probably today's M86, previously M18.