M-13 is an RPO;
RPO = Regular Production Order
An RPO doesn't indicate the end item, but instead a generic description;
That generic description was defined by the vehicle and model year.
From my research, M-13 was used by all divisions for many years to indicate "heavy duty "three speed synchromesh (manual) transmission";
There are the odd exceptions like 1969 where Chevrolet used MC1 to denote specifically which HD-3spd was to be used - in that instance it was the new Muncie three speed.
T-16 should be the BW product code.
regarding my prior post in this thread;
From a little more research my suggested progression appears to be wholly in error...
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1970 Formula 400
Carousel Red paint on Black standard interior
A no-engine, no-transmission, no-wheel option car.
Quite likely one of few '70 Muncie three speed Formula 400's left.
1991 Grand Am: 14.4 @ 93.7mph (DA corrected) (retired DD, stock appearing)
2009 Cobalt SS: 13.9 @ 103mph (current DD; makes something north of 300hp & 350ft/lbs)
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