The very early Singer Sewing machines do not have a letter.
The serial number is usually located on the bed of the sewing machine.
The one exception I can think of is the Singer 222.
It's the free arm version of the Singer Featherweight and there were just a few made. As far as, the rest go they were mass produced machines.
It is nice to have but know that Singer is the only sewing machine company you can do this with.