I am guessing there was a typo, and that 'M12' application was for 1964(?).
The RPO tends to tell a story - the only confusing part is that many people associate an RPO with a singular physical item;
GM reused RPO's year after year, and the RPO was intended to imply the
type of part installed.
The physical part installed could be different over different model years, or different in the same model year - between different body styles.
Divisional differences ad another layer of complication in using an RPO to identify a single part.
M11 or M12 were different RPO's used for a light duty or regular duty three speed transmission.
M13 was an RPO which specified a 'heavy duty' three speed.
M20, and later M21 were used for four speeds;
M20 essentially was intended to indicate a wide ratio four speed.
HERE is a link which may be useful.