The DI indicates production by Defence Industries Limited, a WW2-era Crown Corporation which operated four or five huge ammunition plants in Eastern Canada. One plant made nothing but SAA (Small Arms Ammunition), another made various karger calibre stuff for crew-served light weapons such as Oerlikons and Bofors guns, another made artillery rounds.....
Canadian WW2 production of .303 ammunition was astronomical, which is understandable. What is harder to understand is how they managed to keep their standards so very high.
Your DI casing is a representative of some of the nicest .303 brass ever made for reloading. Originally, it was noncorrosive and nonmercuric, which put it in a class almost by itself at that period. I hoard this stuff, as it is great for reloading.
The Z, as Tiriaq and John Sukey have pointed out, just means loaded with an extruded powder. The powder they used was roughly of the class of 4895.
Defence Industries did not follow british practice by coding the headstamps of their ammunition. They just loaded everything with the same headstamp and then followed US practice and painted the tips of the bullets to indicate anything that was not Ball ammo. Prior to this, all Canadian military ammunition followed the British practice of having the headstamp show what type of ammunition it was, as well as different primer sealers. Defence Industries were the first to break away from this system and go to the now-almost-universal American-type system.
Your casing is not a rarity, but it IS a real piece of history coming from a time when Canada was a major industrial power. What has happened?