Nice one, gear guardian!
I don't have the game so it is cool to hear
interesting details like this. Posts like yours are one of the reasons which
make a subscription to this forum worthwhile. Welcome to the board!
So (according to Lucas Arts), Henry Sr. enlisted for good ol' Uncle Sam.
My guess would be for the home guard reserves, just before Indy was
born, during the 1898 Spanish-American War. That same year there was
also U.S. military presence in Nicaragua, China, Samoa...making the
probablity of him joining a "volunteer corps" at that time seem very likely.
Perhaps he left the manual on Indy's desk after his son had lost a fight
with his high school rival, Butch, from the Young Indy series?