This might help Darrin:
Rob nails it pretty much on the head. As you may know, the Shermans originally had the 75mm gun with its ammo load in bins located in the sponsons. Later on, units in the field and then the manufacturers started to add applique armor patches onto these sensitive areas of the hull.
The glycol filled lockers were developed to be an additional protective measure against fires (and explosions) from pierced ammo lockers. The M4A3 75mm (late 47 degree hull) had these.
All the 76mm gunned tanks had these as well. They also happened to relocate their bins to the tank's floor, beneath the turret basket. This shielded the 76mm shells even a bit more.
The 105mm Howitzer tanks (M4 and M4A3) had old style "dry" racks and no applique armor either.
AMPS 1st Vice President
2008 International Show Chairman
April 10-12, 2008. Auburn, IN