From my research into FN64 turrets, it seems that they were only retro-fitted to the Mk. II Lancasters of 6 Group (and only then when said Lancs had the 8,000 lb bomb doors). These would have been lost to service when Canadian built Mk. Xs replaced them later in 1944. Also, and as stated above, to some Merlin Lancasters of G-H equipped Squadrons in No. 3 Group which a) also had 8,000 lb doors and b) did not have H2S (in other words, quite a small number). I've never seen evidence of any other units fitting them although I cannot discount the possibility.
Just how widespread was the practice of fitting single guns in the belly is hard to tell as they were unofficial modifications and hence probably not recorded. In all my years of peering at every Lancaster photo I can lay my hands on, I don't think I've ever spotted one.
If you read W.R. Chorley's Bomber Command losses for 1944 and 1945, you will come across only a few references to a mid-under gunner being carried.
Most losses with eight crew members on board is explained by them being extra set operators (101 Sqn with A.B.C.), navigators (P.F.F. or 5 Group's own Marker Force) or "second dickeys" along for their initial operational experience, etc.
Don't forget that the Halifax III units responded to Scrage Musik in the same way, i.e. installation of the Preston Green mid-under turret or unofficial lash ups.