These sideviews are based on a British patent application (873,236A) by Boulton Paul from 24 March 1936. Assigned to BP's chief designer, John Dudley North, this patent was published as 515,992A in September 1937. With figure keys edited out, the introduction to that patent goes as follows:
"515,992. Arrangement of guns on aircraft. BOULTON PAUL AIRCRAFT, Ltd., and NORTH J.D. March 24, 1936, No. 8732.. [Class 4] [Also in Group XXI] Guns are mounted on a member projecting a considerable distance from the aircraft body, the member being rotatable about an axis fixed relatively to the body and carrying at the end remote from the body a support for a gun or guns. Further, the gun support is rotatable on the member about an axis at right angles, the arrangement being such that the gun or guns are aimed in elevation and azimuth by a gunner situated within the aircraft body. Fig. 1 shows the mounting arranged at the tail for covering a rear hemispherical field of fire with limited forward fire clear of the fins and rudder [...]"
This patent is better known for its 'dumbell' variation to arm bombers with 4-gun tail turrets. Illustrated here is the original single-gun variant. The 'dumbell' mounted 4 x .303-in Browning guns. The single gun type is not specified. The gun itself is overscale if meant to represent a .303 weapon. Since the BP patent application 873,236A predates the RAF's 1937 adoption of the .303-in Browning as standard, it is possible that the drawing is meant to represent a larger weapon - eg: the Vickers .5-inch aircraft gun.
In the patent illustration, the BP remotely-controlled single-gun mount is shown attached to a rear fuselage and tail which vaguely resembles the forthcoming Defiant ... and so that is how I have rendered it. In reality, I have no clue what type of aircraft J. D. North actually had in mind for this armament. Nor is any mention made of whether the system was to be semi-retractable. I have made that assumption based purely on the awkwardness of landing an aircraft with such a 'stinger' without retracting the gun.
Assuming that this gun system was workable at all, I would see it as better-suited to a modest extension of a more finely tapered rear fuselage - ie: in form, more like the Gloster F.5/34 - to get around the need for such a long tubular mount to clear the empennage.