I had never found a John M. DAVIS in the 18th Cavalry until a few weeks ago. I succumbed to the American Civil War Database listings on the internet. While searching for ancestors, I found a John M. DAVIS listed with the 18th Cavalry. That surprised me, since there was no such listing in the Soldiers and Sailors System run by the NPS.
Once I had found my DAVIS in the 18th Cavalry, I suddenly realized the Company designation -E- was the same as another John M. DAVIS in Morgan's Regiment. Since I knew I had an ancestor in Company I of Morgan's regiment, I looked him up in the 18th Cavalry. He was listed with Comany I of the 18th. Neither man was listed in Morgan's Regiment. I have no idea where the Database got their information.
I then jumped on this Message Board and found a message that stated that many of Morgan's men were previously in the 18th Cavalry prior to joining Morgan. That brings us up to now, and the reason for my query.