Lets step back and look at this in a different light.
The US Government contracted to have firearms manufactured from different manufactures. Regardless of the type of firearm each manufacturer was assigned a serial number block for that model. There were several manufactures for Garands, M1903's, M1903A3's, etc. Now in the 1940's things were a little different then now. So the manufactures over ran their assigned lot of s/n. Not a problem. The Government had that manufacture add an alpha character to the serial number so it could be tracked and not confused with anothe4r. M1icon Garands used an A I believe. Remington used a Z and Smith Corona used a C. I forget the other alpha characters that were used.

So, the alpha character does not denote the model of the firearm. It simply segregates the serial numbers so they are not confused with another s/n in the system.
Stop over thinking this.