Is there any information available how the transition from "985" case to "742" was actually done?

Searching from the Internet brings this info:
"1-3/4" large stem pinion (aka. '742')
Carrier casting numbers: 1634985 (1957-1964), 2070742 (~1961-1969)."


Were they both cast during the same period of time (different foundrys?) or were they just using the remaining stock of "985"'s? Is there any production plant dependencies (eg. some plants were using just one casting number? Model dependencies (eg. available on just certain models?

Any reason why my December 1962 built MW Savoy should have one or the other?

The research I've done shows the same conflicting info and the parts book is no help; it shows the same part number for large-stem carrier w/caps (3-03-3) from '57 up through '68. The introduction of the 489 changed the part number.

I've been told that the only difference between the 985 and the 742 is the material, cast steel versus cast iron, so I guess they saw no reason to change the part number.