Directly quoted from the Surly website FAQs | The Information Hole | Surly Bikes:

"You know that serial number on the underside of your bottom bracket? No?! Ok, so flip your bike over and take a look closely at the bottom bracket shell (the cylinder at the bottom of the frame around which your crank rotates). There will be a code stamped into the shell starting with the letter "M". Sometimes this stamp has a bunch of paint that has collected in it but look closely, it is there, I promise. The following number after the M signifies the last digit(s) of the production year of your frame. So, for instance, M2598144 was made in 2002, M8128666 was made in 2008 and M10034269 was made in 2010. Now write down the serial number and keep it somewhere you can find it in case your bike gets stolen. This might be the only identifiable mark on your frame if it comes back to you!"