First I apologize for the poor inflection in some of my words in my first post.
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.)
That answers another question I asked: When were the busses renumbered to match the streets. I also posted a list of renumbered crosstown routes for others' reference:
M13=M8
M26=M23
M16=M34
M106=M42
M27=M50
M28=M57
M29=M66
M30=M72
M17=M79
M18=M86
M19=M96
M19=M106
M20=M116
Now the M16, M27 and M30 are still there thanks to them being branches. And the M13, M19 and M20 were renumbered later. The M19 had both a East 96th and 106th branch that now have separate numbers.
The list shows even OLDER numbers for some of these routes. Like 20 for 57th Street, M3 for 50th, M4 for 79th, M5 for 86th, M6 for 96th and 106th, M7 for 66th, M16 for 8th, M17 for 14th, M18 for 23rd, M19 for 34th and M102 for 125th (current Bx15).
Except for the last one, for what was the earlier renumbering done?