Most interesting. I agree one should always see the original record if there's any chance of it. You got this from the History of Leiter Hospitsl at the link you gave in your first message, right?
The family thought he was a Doctor in the Spanish American War, but I doubted it given how young he would have been (21 as you note). Something's a little off as he went on to claim the title of MD later -- I think the off thing is Justus himself.
His father was supposedly a druggist in Baltimore and Justus supposedly trained for that profession too. I haven't found any educational record for him, just the claim of MD from some sources.