Canadian 1964 Pattern Grenade Pouch. It was designed to carry 2 hand grenades* on the inside and the rifle launching adapter outside with the Velcro strap. 

Part of the Canadian Forces 1964 Pattern Web Equipment (WE'64).

There are 2 variations. Canvas and Rubberized canvas (see comparison photo).

It measures 4 1/2" x 4" x 2 1/4" with a 2 1/2" belt loop.

*The grenades and launch adapter in the photos are for demonstration purposes only. They are not available for sale.