You are correct about the model years of C4 and C6. However, I don't think that you find that explaination in any internal Ford document.A booklet was given to us at work on the 50th anniversary of Ford automatic transmissions.
It indicates the Simpson gear set design were of the "C" family and the earlier BW licensed design were of the "X" family, i.e. FX, MX, HX, FMX were of the Raxigneaux design.
This is why the C3 and C5 don't correlate to 1963 or 1965.
Incidently, the original code name for the C4 was XP transmission, and the C6 was XPL transmission.
Regards,
Dennis K.