This postcard shows 3 Ontos, M50A1s, training at Camp Pendleton CA.
The Ontos was a US tank destroyer used by the US Marine Corps. It was produced from 1955-57 and in service from '56 to '69. It mounted 6 106mm recoiless guns, 3 on each side of the turret. I was surprised to learn that the crew loaded the guns from outside!
some of the barrels had a .50 cal gun mounted with to help aim, not like a tracer round, but the puff of smoke where the aiming round landed told you where the 106mm shell would land. It appears to have been used against infantry, too, and small wonder--6 x 106mm shells landing around would certainly neutralize/disperse/kill men, trees and buildings.
Besides having to load the guns from outside the tank, only 18 rounds could be carried (on the outside) and it had a high profile given how the guns were mounted.