Canadian ammo. It is non-corrosive, boxer primed and reloadable, but the primer is crimped in.
There are two stories about the ammo. The one I heard as a kid at Interarms is that it was made much later than the dates on the headstamps for use by the CIA. Interarms was a CIA proprietary originally.
The other story about it was that it is just Canadian Besa machinegun ammo used in WWII tanks.