1971 interview detailing the original decapitation ending
Cinematographer Conrad Hall: "The ending in the script had them find two people there when they arrive, two Japanese men, and they want to do Marvin in right away, but Mifune says no. So one day they hear gunfire on the other side of the island, and Mifune goes to see what it is; when he comes back, Marvin is lying in the compound decapitated, and these two guys are sort of sheepishly grinning, knowing they had done something wrong, but kind of proud that hey had killed the American. And Mifune, in his rage, grabs the sword and decapitates both of them. That was the ending in the script."
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Much more firsthand information on the scramble to devise an ending to this film photocopied from 1971's 'The Art of the Cinematographer : A Survey and Interviews with Five Masters by Leonard Maltin' at link below:
https://imgur.com/VJZNklR