Not sure. Dad taught Semie Moseley to play the Guitar. He learned on Grandpa's 1952 Les Paul gold Top Gibson.
Every time Grandpa would go through Bakersfield, Semie would True up grandpas guitar, adjust the neck clean the body change the potentiometers I remember once it when Semie got Grandpa's guitar out of the trunk of our car. And while he and dad were talking Semie took wire cutters and cut all the strings off of Dad's Les Paul guitar and filed down the frets to make it easier for grandpa to play.
It wouldn't surprise me if Semie changed to accommodate a six string instead of a 12. I remember grandpa talking about a 12 string.
Around that time an amplifier had fallen over and cracked the neck on Grandpa's Les Paul.
Grandpa took the guitar to Semie to see if he could repair it. I'm not sure what city Semie's factory was in at that time but Grandpa and Grandma was within driving distance.
Grandpa's Les Paul had a mahogany neck on it. Semie used a different type of wood for his necks. To order The mahogany neck would've taken time, and dad needed to get term for his ministry.
Semie was making this style of guitar when Grandpa stopped in to see Semie. He built out the 6 string for dad.
I don't know if Semie changed out the neck or just used that body and added the 6 string neck to create Grandpa the guitar you have there.
Semie would do anything for Grandpa / Grandpa would do anything for Semie. They had a wonderful relationship.
Maybe that might collaborate with what you're thinking?
I also asked him about the M and he said this too, which is pretty much what you were saying.
At that time, Semie was going through a bankruptcy and couldn't use the name Mosrite. He used his personal name, Moseley. No 'M' or Mosrite logo.
I'm a bit nervous to destring the guitar, but I'm sure that's a lot easier than taking the neck off the Mosrite, lol!
Also, my father says Danna had in the last few years looked at the Mosrite's internals and he says he's trying to get the information for me. I think I'll wait for that before I event attempt removing the neck.
My friend says the serial reads A0893. I still haven't gotten a rubbing since I only have a mechanical pencil. I'll pick one up sometime soon.