Several of them list the marriage date as June 6, 1876. I don't know their source. Marriage licenses were not required in Pennsylvania until 1885. The only sources for marriage dates before then are usually church and pastoral records, newspapers and sometimes Civil War pensions. Since Francis was a doctor he may have had a biography written about him in a county history book. You could check with the Mercer County Historical Society or the Crawford County Historical Society in case no one can cite you the source.