I am wrong on this point. Apparently there was a head of Bush in the prop room that was used. Big deal. It begs the question, though, “Who were the OTHER heads used?’ Could there have possibly been an Al Gore in there, or a Barney Frank?
Who knows where this may lead.
I see no reason to believe their “it was a prop lying around” story. In what previous HBO production was such a prop created and used for?
More likely, they either created it especially for this “easter egg”, or if it was lying around, it was probably as a macabre decoration in the collection of some lefty associated with the show.