I went into this topic (i.e. the under gun mounting) in some detail while researching at The National Archives many years ago. The facts contained in several Bomber Command files that I studied are at odds with what you tend to read in books published since the '60s (and, by extension, what you read in threads such as this.).
Bomber Command had actually picked up on the increase in attacks from beneath as early as the summer of 1942, almost as soon as the upward firing guns were trialed by the Luftwaffe. While the nature of the attacks was recognised, the means of delivering them was at this time thought to be from a steep angle of attack by the night fighter (which the majority probably were, suspicions about upward firing guns came later).
Harris attended a conference at the Air Ministry in June,1943 to discuss improvements to the under defence of bombers. The meeting was mainly about improving the field of view (particularly downwards) from the tail turret. This would eventually lead to a proposal by Fraser Nash for a two gun adaptation of the existing FN20 rear turret known as the FN220 which ultimately came to nothing.
As an interim measure, a scheme was devised by the Bomber Development Unit which consisted of a single .5" Browning as illustrated above by longer ron, utilising the original opening intended for the ventral turrets which had already been deleted. It was ackowledged as being a "lash up" right from the start. There was a version for the Stirling, Lancaster and Halifax and there was to have been a version for the Wellington but I was never able to establish if this actually went ahead. The requirement was for all aircraft of the Main Force to be equipped and also those of the Conversion Units.
Of the three installations, the Halifax was considered to be the least satisfactory and it was re-worked into what is referred to as the Preston Green mounting. I cannot say this with 100% certainty but I have been unable to find any commercial firm with such a name - it is however interesting to note that the Handley-Page representative at project meetings was a Mr. P.R.T. Green - perhaps he got given the job of improving the original installation?
The whole scheme was meant to go ahead with the usual energy and priority (Harris was probably thinking of those long winter nights on the road to Berlin and other Eastern German destinations). Installations were made on the bomber production lines and conversion sets were delivered straight to the Groups. There were some delays in obtaining the necessary conversion parts (certain parts from the U.S.A.), and some Squadrons with Mk. II Lancasters initially received the wrong conversion sets, but by the end of 1943 the work was going ahead. Each Group nominated a base for the work to be carried out. I have seen communications from the Groups indicating that the work was largely complete by the spring of 1944*, however this coincides with the period when H2S was being installed so it must have been a particularly hectic time on those airfields, gun one week, radar set the next!
In the end, and as already pointed out, the installation of H2S took priority over the under gun. Some surviving aircraft and left over conversions sets were passed on to 3 Group (for use on aircraft without H2S) and some were still in use at the war's end.
That's about as far as my research took me. What's missing from the narrative is just how much use was made of the installation by the Squadrons, i.e. were they enthusiastically received and put to use or just treated as another rush job with little practical value? I've seen individual accounts of an eighth crew member being taken on ops, sometimes a "spare bod" gunner and sometimes even a volunteer from ground crew, but I've never come across a comprehensive report from the end-users which perhaps helps explain the general level of ignorance about this topic.
* All those Airfix/Revell/Frog/Hasegawa Lancs, Stirlings and Halifaxes on the modelling forums which ought to have the gun but don't!
Last edited by 682al; 26th Jan 2021 at 13:17.