Quoting Tony Buttler from the mentioned BSP volume:
" ..the tail defence consisted of Boulton Paul's patented 'Dumb bell' type mounting which housed
two Brownings to either side (total four) with the gunner housed in the rear fuselage. This arrangement,
which was produced as a mock-up, placed the guns in stub winglet fairings in an attempt to reduce the drag
penalty of a completely moving turret..."