1955      M11           B-0-P L-D 3-speed (5-bolt top cover)

1956      M13           B-0-P H-D 3-speed (6-bolt top cover)

1957      M13           B-0-P H-D 3-speed (6-bolt top cover)

1958      M11           Early Muncie L-D three-speed
   M13           B-W T85 H-D 3-speed
1959      M11           Early Muncie L-D three-speed
  M13           B-W T85 H-D 3-speed
1960      M11           Early Muncie L-D three-speed
   M13           B-W T85 H-D 3-speed
  M20           B-W Early T10 L-D Wide Ratio 4-speed (Chevyshort tail) – Released Jan 1960
1961      M11           Early Muncie L-D three-speed
M13           B-W T85 H-D 3-speed
  M20           B-W Early T10 L-D Wide Ratio 4-speed (Chevy short tail)

1962      M11           Early Muncie L-D three-speed
  M13           B-W T85 H-D 3-speed
  M20           B-W Early T10 L-D Wide Ratio 4-speed (short tail)
M20           B-W T10 L-D Wide Ratio 4-speed (long tail)
1963      M11           Early Muncie L-D three-speed
M13           B-W T85 H-D 3-speed
M20           B-W Early T10 L-D Wide Ratio 4-speed (short tail)
M20           B-W Early T10 L-D Wide Ratio 4-speed (long tail)
M21           B-W Early T10 L-D Close Ratio 4-speed (Chevy short tail)
1964      M11           Early Muncie L-D three-speed
M13           B-W T85 H-D 3-speed (Early '64)
M13           Ford H-D 3-speed Top Loader (Late '64)
M20           B-W Early T10 L-D 4-speed (Chevy short tail - Early '64)
M20           Muncie L-D 4-speed Wide Ratio (1st Design -Late '64)
M21           Muncie L-D 4-speed Close Ratio (1st Design - Late '64)
1965      M11     X    Early Muncie L-D three-speed

As you can see, it was quite confusing in the early '60s with the long
vs short-tail trannies. In addition, 1964 was the changeover for PMD and it was not without
problems.  Muncie production could not keep up, so Pontiac continued using the T10s in the big
cars.  Unfortunately, PMD also used the same UPCs (M13, M20, M21), regardless of the type of trans actually installed...

Another point of interest was transmission torque capacity.  Pontiac's
use of Light-Duty (L-D) and/or Heavy-Duty (H-D) was very misleading during these

early years (L-D

The Early Muncie was a 1930's design with a drum syncho and quite
fragile.  This trans routinely
broke behind docile 95 hp Chevy 6's in pickup trucks.  So using it
behind a V8 was definitely not
a brilliant idea.  The B-O-P trannies were actually pre-WWII designs
with straight-cut gears (hence
the horrible gear noise).  However, straight-cut gears are very strong
(ever hear of a Jericho?).

Anyway, the 1956-57 Pontiac H-D three-speed was stronger than any
other 3-speed ever mass
produced, with the exception of the Ford "Top Loader".  A T85,
although much stronger than a T10,
is not quite as stout as the big old Buick unit shown below, but
they're a hell of a lot quieter...

By the same token, the Early T10 was designed for the 283 Chevy.  It
was never meant to
cope with the brute torque of a much larger Pontiac V8, Chevy 348-409,
or Ford's 406-427.
The 427 motors really did the egg shell trick on T10s, because they
could wind over 7,000
rpms.  Remember, force increases exponentially with RPM...

In Muncie land, they also had their problems.  With the release of the
Rat, 1st design
Muncies could not cope.  GM went to 2nd and even 3rd Designs (M22),
but the Muncies
would never be as strong as the Ford or Chrysler New Process with
their steel cases.
Because of their aluminum cases and the intermediate plate, I've
always considered
M20/M21s to be a light-duty trans, and quit using them for drag racing
in the early 70s
Super Stock wars...