The Terrapin looked different from both the DUKW and the AMTRAC. It needs its own pic and icon. I think it was a soft skin amphibian.
I also think the British used DUKWs as well. At least they did after WW2 because my Uncle Eamon used one in Malaya to hunt down communists.
Must go now. I'm late for the pub. I may post some further info, sources etc when I have some free time.