The rule gives one year.
The rule
One continuous number, no letters. The year is the one in which the highest serial reached that year. Last serial of that year: 58679.
Numbers below 8349 are 1898 or earlier. In 1956 the two main sources disagree on the year-end figure (152775 or 153225), so serials near 152775 to 153225 could fall in 1956 or 1957. Serials 900001 to 902908 are Sigma-Martins from 1981 to 1982 and the table does not apply to them. Martin dates by the year-end figure, so a guitar made in the first weeks of a year can carry a number near the previous year-end.
Usually found
On the centre block inside the body, visible through the soundhole. Also stamped on the neck block.