1953 Pontiac T46E1 Otter Amphibious Carrier.

Designed by the Pontiac Motor Division in the early 1950’s as the T46, this vehicle began life as the intended replacement for the Studebaker M29 Weasel.

The Otter was in service with the United States Marine Corps (USMC) who deployed the vehicle in Vietnam with the first 33 vehicles arriving in Danang in 1965.

https://www.warhistoryonline.com/news/vietnam-3.html