and serial numbers were added, beginning with #8000.
First, they stamped the logo on the back of the peghead (from circa 1895).
Then they started putting decals on the front of the headstock in 1930.
So if you don't have a decal, it's pre-1930, if you have both decal and stamp, it was built between 19.
They also used bar frets until 1934 (and probably on some ukes a few more years) after which they used "T" frets." Mike Longworth, Martin Company historian for 25 years added, "...
I found what I believe to be the initial order for decals at Martin. Remember that during the depression, things might set around in the factory, sometimes incomplete, for a while waiting on someone who had the money.
Since serial numbers weren't introduced until 1898, approximate dating of the earlier guitars can be accomplished by knowing when Martin made changes to their labels or to the brand, which can be found stamped into the back of the head, the upper part of the back, and just inside the soundhole on the long strut which reinforces the back joint. Ukuleles didn't have serial numbers, but Craig Thompson wrote, "I recently talked to the ukulele expert, Mike, at Gryphon Stringed Instruments in Palo Alto.
The very earliest Martins, around 1833, had a paper label "Martin & Coupa". One good way of dating old Martin ukuleles is the headstock.
Some of these turn up and vary from the standard instruments, but they are usually very nice and are frequently signed by the maker. Martin workers are allowed to build a guitar for their own use after they have been with the company a stipulated period. Martin also built mandolins (introduced in 1895), ukuleles (introduced in 1915), taropatches (an 8-stringed ukulele), tiples (a 10-stringed ukulele), Hawaiian guitars, and tenor banjos (in 1923) plus a few custom instruments. Zoebisch & Sons was the exclusive distributor of all Martin instruments, and that name appears on a paper label inside the sound hole. Beginning in 1929 the model numbers were marked on the neck block, inside the body. The size and style designation was written on a paper label, glued inside the top of the instrument case. Some pre-1898 guitars have the date penciled on the top just inside the sound hole.