Interesting juxtaposition of photos showing the markedly different location of the mid-upper turret on those two marks of Lancaster.
The 'Martin' turret on C-GVRA is actually only complete above the fuselage, so access inside the aircraft, and more enjoyably, inside the turret in flight, is easier.
If I've got my turrets right, the mid-upper Frazer-Nash in PA474 actually came (back) from Argentina...
Regards,