I know just about nothing about Hydra-Matic transmissions. However, over the past few years, I have obtained some Chevrolet & GMC manuals that cover various mid-50s Hydra-Matic transmissions (because I am restoring a 1954 Chevrolet 1/2 ton truck that has a Hydra-Matic transmission).
Below, are a few photos of the cooling unit in a 1955 Twin Hydra-Matic transmission (used in 620-950 series GMC trucks). This transmission was basically two four-speed Hydra-Matic transmissions mounted on top of each other (and could have come equipped with a 2-speed or a 3-speed reduction unit). A 21 forward speed Hydra-Matic drive-line must of been a thing of wonderment.
I do not know what sizes of trucks with Hydra-Matic transmissions used a cooler in the oil pan but maybe the photos below will look like the transmission in question.
http://1954advance-design.com/Web%20images/Twin-H-M-cooler-1.jpghttp://1954advance-design.com/Web%20images/Twin-H-M-cooler-2.jpgI also have a few pictures showing a similar cooler-unit in the pan of a 1954 4-speed model 254 Hydra-Matic.
