It was indeed very common. If you look at the signatures on the Declaration of Independence, a large number of them were written as abbreviations: Wm, Robt, Geo, Thos, Step, Jam, Benj, Abra, Jn or Jno. Thomas Jefferson signed as "Th Jefferson."
As for why, I speculate that since everything was written by hand in those days, people just got tired of writing out whole names when it wasn't necessary. Many names were so common, everyone understood what the abbreviations stood for.
This message was edited 12/7/2019, 11:39 AM