I am putting together a historical presentation about a late-19th / early 20th-century U. S. ruralite who was involved in many things including operating a general store. I have seen the abbreviation "Dr." on an invoice after the name of a business which sold Kodak film to him April 3, 1945. Then I see "Dr." after the fellow's name on invoices that were to be issued by the fellow himself. In these cases, what is the meaning of "Dr."?