That is going to depend on which "map" the square is located. The system is explained here:
http://www.echodelta.net/mbs/eng-overview.php
But several of the maps have a "Y" location. wY9520 (on the French Lambert Zone 2 map) is near Grenobel, for example. But other maps also have a "Y9520" location. bY9520 on the French Lambert zone 3 map is in the sea south of Toulon.