Most Martin guitars made are “flat top” models. That is, they have a round sound hole in approximately the center of the flat top of the guitar, with a “pin” style bridge. Martin also made some vintage instruments like archtop models during the 1930s. These can have a round sound hole, or two “f” style sound holes (one on each side of the top of the body), and have an arched top, with a “trapeze” style bridge. Martin also made ukuleles. If a guitar only has four strings (and is not a ukulele), this is known as a Tenor guitar. Uke size instruments with ten string are Tiples. Uke size instruments with eight strings are Taropatches. Martin also made mandolins, which have eight strings. To summarize:
Martin flat top guitars were made in various sizes. The bigger the vintage instrument body, the better and more collectible the guitar. This is why guitar body size is so important to identify on a Martin flat top guitar. Starting in October 1930, Martin stamped the guitar body size right above the serial number inside the guitar. This makes identifying body size on October 1930 and later guitar very easy. Body sizes, pretty much from smallest to biggest, include O, OO, OOO, OM, D.
Nearly all Martin instruments come in different styles. The higher the style number, the more fancy (and collectible) the instrument. Again starting in October 1930, Martin stamped all flat top guitars with the style number, directly after the body size (and above the serial number). Style numbers can range from 15 to 45. A letter can follow the style number too, giving some additional info about the instrument. For example, a “T” after the style number indicates a Tenor guitar.
Originality of an instrument is very important. Modifications (any modifications), are a bad thing in the eyes of a collector. This will greatly influence value. Modifications can often be determined by looking at the model specs for a particular year guitar in this web page, and compare to your instrument. On flat top martins, the most common modifications are a replaced bridge, replaced tuners, or replaced frets.