The 'Normandy Cowl' is the deflector that Cromwells had on the rear of the tank to deflect the exhaust plume backwards. As built, the exhaust goes out vertically, not very tactical amongst the hedgerows!
It is often written that the cowls were to prevent the exhaust from being sucked into the hatches and gassing the crew when the tanks were lined up. I do not accept that as the likely origin of the cowl though as no air is drawn through the hatches, unlike Sherman and directing it backwards merely gasses the crew behind.....
I believe it to be for tactical reasons in that the exhaust plume, particularly on starting up is very visible and gives away your position.
Initially, the deflectors were 'home made' from anything available, I have even seen pictures of cut in half jerrycans used. Eventually, the official cowl was adopted and starts to appear in action by late summer 1944. Comet had one from the start, though with a slot in the centre for the gun when in it's crutch.