In the 1920 census, MacFarland Long is a postal clerk, age 29, living in East Carroll Parish with his parents, Dr. William Long, 56, and his unnamed wife of the same age.
In the 1930 census, MacFarland Long is a Deputy Sheriff, 37 this time, living in East Carroll Parrish with a lodger named Walter E. Treffry.
Dr. Long was born in West Virginia, and his father was born in Maryland. I don't know how this squares with the family history of the better-known Longs.
I couldn't find Mac in the Social Security Death Index. Maybe he retired to another state.