Walter Botts, is the real Uncle Sam. You can see him here where he imitates his poster in an image of 1970 from David Dreiner. Botts was the model for the famous poster, made by J. M. Flagg, with the iconic "I Want You" statement.
At the beginning Flagg used his own face as Uncle Sam, until he was asked to finalise it before WWII. He then decided to use Botts for this model of what we all recognise immediately as American. Botts was chosen as he had "longer arms, longer nose and bushier eyebrows".
More pics of Botts:
And finally the iconic poster
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