The Canadian Lancaster is a Mk.X, it had a Martin dorsal turret with two .5's. It was also set further forward in the fuselage. The British BoB Lancaster is a Mk.I. There is another in Britain that is getting close to being airworthy, which is a Mk.VII. That fairing around the Mk.I dorsal turret is actually a track which prevented the guns from blowing off part of the aircraft when fired --- see the photo attached. You can see two rods with wheels on the ends just under the guns, the wheels run on top of the track.
My Dad served with 617 Sqn ('44-'46) and when I took him around the Nanton Lancaster (a Mk.X), it was interesting to watch him as he walked up through the fuselage, he would say things like, 'we never had those' pointing to some bit of equipment, or 'that was much further forward on ours' ---