This confuses a lot of people who think their style 18 guitar is a style 28. On round hole martin guitars, the serial and model numbers are stamped on the neck block inside the instrument.
Model Numbers stamped above the Serial Number starting in 1930. Prior to October 1930, the model is NOT stamped on the neck block (you have to figure it out yourself! Right: The model and serial numbers, as seen through the sound hole of this 1944 D-18, serial number 90067.
Starting in October 1930, Martin also stamped the model number just above the serial number. Notice the "1" in the "18" does somewhat look like a "2".
These serial number apply to all Martin guitars, flat top and arch top.
It does not apply to ukes (except for the first year, they do not have a serial number).
This shows all of the serial numbers of Martin’s guitars since 1898!