Originally Posted by
Downwind.Maddl-Land
That was my reasoning and assumption too; I wanted to try to elicit from the readership a definitive answer or possibly a reference, as my research had drawn a blank thus far. However, an obscure (Canadian) blogsite stated that the Mid/upper gunner manned the FN 64 (when fitted); I find that counter-intuitive/hard to believe as the Mid/Upper had a far better field of view and was a more effective adjunct to the crew's survival prospects than manning the FN64 with its restricted FOV and utility!
My understanding wrt the Preston-Green fitting ('turret' is far too grandiose a term!) was that it was a late-war retrofit for Main Force Halibag IIIs and VIs that were not universally equipped with H2S, this equipment being primarily destined for 8 Gp and other Pathfinder-designated units. As a predominate operator of the more effective Halifax marks, No 6 (RCAF) Gp was, of course likely to be the primary recipient/adopter of this fit, hence their closer association with it. My understanding is that the P-G fit was retrofitted to those airframes in lieu of H2S (ie in the vacant ventral position), rather than H2S displacing the P-G installation. Willing/Stand to be corrected though!
I think over enemy territory it wouldn't have been feasible for the MU to man both.
My understanding is that the fitted P-G was removed as H2S became available.