Archive papers from Gov Flecther's records
This letter dated Dec 25, 1865 refers to Col B.W. Lewis while Lewis is still alive.
So to narrow the time frame from primary contemporary documents he is Mr Lewis on Nov 2 1864 and Col Lewis on Dec 25, 1865. There is no indication he ever directly served in a military unit so I'm thinking this is an honorific title. His brother, a legitimate Major, never refers to Ben with the rank title.
Interesting person and times though, good story!