My understanding was the B.I Lancaster had Rolls Royce engines and the B.III had Packard built Merlins, in the next few years engines got mixed around, add ons like Different propellers and Bomb aimers blisters changed and were mixed and matched on both B.I and B.IIIs which makes it very differcult to identify just by looking at a picture.
I thought the B.X was designed for the Tropics but never ended up in that theatre, Canadians of course did their own thing a threw in a Martin upper turret.
.50cals and ammunition are a lot heavier in the long run, they only used .50cals in the Rose? rear turret on a Lanc, I do believe most gunner kills were from the rear turret?
My grandfather did 40 missions mainly in B.II which he much preferred over the Merlin versions.
Andy