Nope, LONGBOX55 - typo error - The owners manual for '51 shows 6 pints (not quarts) total capacity.

The capacities I posted above for 1954 are all wrong - for American trucks (I was mistakenly looking at a Canadian Shop Manual - Imperial quarts/pints/gills/etc).

The USA 1951 (and 1954) Shop Manuals show 6 pints for the 4-speed and 1 1/2 pints for the 3-speed transmissions.