Is Yamsk 11 based on a real life location?
According to the Russian Full Metal Panic wiki's Yamsk-11 article (Янск-11, see reference), it says that Yamsk-11 is here: 60° 8' 10.66" N 153° 54' 20.60" E (East Siberia). "Yamsk-11" is not the name of the city, but a postal code for mail delivery. There is a city, Yamsk, 63 km to the south of "Yamsk-11". It was common practice in USSR to give a postal code name with a number for closed towns. Apparently, there was a town at those coordinates that's been closed. For another example of this, see Wikipedia's page for Arzamas-16 or Chelyabinsk-65/40.

So looks like there is a real location but not sure if it has anything in common with the fictional "Yamsk 11" in FMP (but who knows), and the name "Yamsk 11" is a postal code from the USSR days.