From the quoted source by Crismon, the XM521 was built by the Detroit Arsenal and used a Willys AO-164 horizontally opposed 4 cylinder 105 HP from 164 CID. The 5250 pounds empty was less than half the empty weight of either M34 or M135 for the same useful load. It is stated that it performed well cross country and swimming. Even by the standards of the day the truck wasn't highly powered (and the engine wasn't a large displacement with high torque but low horsepower design) so I wonder how it did on the highway. It had 2 12" propellors for water drive when built, I didn't see them in the photos of the sale vehicle. It was a test vehicle, not sure if there were more than one made or if this is the "one and only".
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