M22 production did indeed begin in 1965, my error and I should have known better since the M22 as offered in 1966 Chevelles.
I did get most of this information from these two pages, http://www.5speeds.com/muncie2.htm
so if they're wrong then I'm wrong but I'm certainly willing to make corrections.
As some of your information seems to be in conflict with them (and all I wish to know is what's correct, not who's correct), please excuse me if I ask a few questions.
Since the M22 was introduced in 1965, what main case casting was used for 1965 since you indicate 3885010 was used in 1966-67 only?
Approximately when in 1970 did the 10 spline input get replaced with the 26 spline input since both are stated for 1970? Or was the 10 spline input used with all non-LS6 1970 Chevelles and the 26 spline used only with LS6 1970 Chevelles?
What main case casting and tail housing casting was used in 1963 if not 3846429?
I think the date issue is described correctly in the paragraph
In 1967 and 1968, the date code got a year designator and a letter designator for the month such as P8A01, meaning 1968 (8), January (A) 1st (01). Note that a Muncie dated with a September to December build date was actually built the prior calendar year. An example would be the date code P8T13, meaning 1968 (8), December (T), 13th. The transmission was assembled December 13, 1967 for the 1968 model year.
This is meant to explain the year designation in a date code is the model year, not the calendar year - emphasis added here for clarity.
Do you have a suggestion for a better way to describe it?
As always, input and corrections are welcomed. I'm not a Muncie guru or specialist by any means and (generally) take other sites at their word unless I know it from other research to be false. I doubt if there are any "official" GM or Muncie documents on things like casting numbers and data are arrived at through years of looking at original stuff and I trust experts in areas like this for correct information.
Appreciate the help!