It is worthwhile to note that the M carried on Brighton and 4th Avenue-Nassau Banker's Specials pre-Chrystie had nothing at all to do with the post-Chrystie M services, all of which involves Myrtle service in some way.
The M was intended for what used to be called the Myrtle-Chambers service from day one but was used for the Banker's Special as a matter of convenience. At that same time, the Eastern Division used only numbers, so the Myrtle Chambers was 10.