A person who assaults any person having lawful charge, control, or custody of any mail matter or of any money or other property of the United States, with intent to rob, steal, or purloin such mail matter, money, or other property of the United States, or robs or attempts to rob any such person of mail matter, or of any money, or other property of the United States, shall, for the first offense, be imprisoned not more than ten years; and if in effecting or attempting to effect such robbery he wounds the person having custody of such mail, money, or other property of the United States, or puts his life in jeopardy by the use of a dangerous weapon, or for a subsequent offense, shall be imprisoned not more than twenty-five years.