This postcard was mailed from Fort Bragg North Carolina in April of 1940 although the car pictured in the background makes me think it's a bit older. It's captioned "New Anti Tank 37mm Gun" and printed on the back is this information: "The old style 37mm gun, firing approximately a one-pound shell, had a barrel so short that it was often termed a howitzer. As developed into the new anti-tank weapon shown here it is long of barrel, mounted on pneumatic tires for mobility, protected by a shield and equipped with latest sighting mechanism." The picture makes me wonder if it did indeed have a 2-man crew.
Most likely the same gun, a 37mm, as seen from the front on this Camp Rucker Alabama postcard from c.1940. A more realistic crew of 5 mans this one.
Notice the skids lowered on this one; on the upper image they may be raised above the tires (see the curved piece above the tire in the first image.)
This final postcard shows a "Jeep crossing rough terrain pulling 37mm anti-tank Gun." Wouldn't the end of the barrel be covered to prevent damage?
