It's a fascinating topic.
Despite what Mr Dyson says, his colleagues had detected the rise in night fighter attacks from below (and rear) as early as mid-1943.
Urgent action was taken to counter the threat. The solution decided upon was the 0.5" Browning "lash up" devised at the Bomber Development Unit (not by "the Canadians"), using the aperture originally intended for powered turrets in all the main bomber types still in service, i.e. Stirling, Halifax, Lancaster and Wellington.
Trials were conducted on the three heavies (I'm not sure the Wellington ever had the modification in view of its imminent departure from Bomber Command). The Lancaster's installation was judged to be slightly superior to that in the Stirling, while the Halifax was judged the least successful. The latter installation was modified, producing the "Preston Green turret" which was only a fairing in reality. I've searched high and low for background information as to who/what Preston Green was. The closest I've come so far is that the representative for Handley-Page in these trials was a Mr P. T. Green (co-incidence?).
There were inevitable delays in getting the aircraft equipped, but by early 1944, all the three types were being modified, either on the assembly lines or on the units.
The major problem, of course, was the parallel drive to get H2S into the same aperture in the airframe. Faith was to be pinned on H2S and Fishpond to detect the night fighter. That didn't work out too well either.
No. III Group continued to use the under gun lash up after the other Groups because their Lancasters had the 8,000 lb. bomb doors and no H2S fit.
Meanwhile: rejected by Fighter Command in 1942...