Complex sentence: His first position there, which he retained for three years, was as postal clerk, and after an interval of about a year, during which he ran a stage coach between Tucson and Silver Bell, he served as assistant postmaster for four years.
Split into simpler sentences: His first position in Tucson was as postal clerk, which he retained for three years. Then he he ran a stage coach between Tucson and Silverbell for about a year, then he served as assistant postmaster for four years.