Afirm, it's a map refeence :
complete on is : 14OstN BE3, see here :
http://www.lesbutler.ip3.co.uk/jg26/gradnetz6.gif
Square is BE, 3 represente third sub divison of BE square, left up corner.
In fact BE square is divided in 9 sub divissions :
123
456
789
So BE 3 represente a south of Paris position.
and here, a map of Arpajon:
http://www6.mappy.com/sidaWnpX2+PLJn...m1=250&x=0&y=0
Hope it's help.