There are few American weapons built during the Second World War which did not serve with the US Armed Forces in some way, shape or form. Even the P-63 Kingcobra had a number of aircraft serve as gunnery trainers. (There are others, such as the Martin Maryland and Baltimore, but those were designed for overseas sales and little used by the USAAF.)
The Staghound armored car is one of the few that was built and accepted for service with the US Army, but never used or wanted by them and thus nearly 100% of its considerable production run went to the Commonwealth for use as a heavy armored car. The US Army instead opted for the lighter M8 6 x 6 Armored Car and its companion M20 6 x 6 Armored Utility Car as they fit the US Army model for cavalry scouting vehicles. The US Army considered reconnaissance more important than combat, whereas the Commonwealth doctrine saw heavy armored cars used to engage light targets and provide infantry support.