Depends on where you're looking. If on the transmission M would indicate the Muncie plant. If on the vehicle ID plate, M would indicate a 3900.
Here's the info from the 55.1 Vehicle Info Kit on serial numbers.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
