Forgot the 15F, this was a 10 series with a factory cargo door so airlines could convert them and fly pax during the day and cargo at night. DAL did this for a while in the late 60's turned out to be a flop. A 15F has two over wing exits, the 10 only has one over wign exit. A bid of trivia from the cargo world.
The -10's and -15's were referred to as the 2 door and 4 door models respectively.
DC