Is there any confirmed documentation on the origins of the sake bomb?
The sake bomb or sake bomber is a beer cocktail made by pouring sake into a shot glass and dropping it into a glass of beer. (Wikipedia)
Take a look at this article: WHERE THE HELL THE SAKE BOMB CAME FROM: A LESSON IN IRONY
From the article:
A few sources suggest that sake bombs were actually invented by American soldiers occupying Japan in the years following World War II.
The "sources" mentioned are this additional article at Los Angeles Magazine: An Ode to the Sake Bomb