I don't know, but my guess is that the slot is not mil spec.
http://en.wikipedia.org/wiki/Picatinny_rail
EDIT: Upon further research, it looks like the rails with a slot cut into them would not be standard according to NATO Accessory Rail STANAG 4694 spec. New spec uses the whole top surface as mounting interface.
http://upload.wikimedia.org/wikipedi...E2%80%9D_2.jpg

