I was pleased to see a report [here] on the rebuild of the Australian War Memorial's Lockheed Hudson Turret, a rare example of Boulton Paul's work.
It is a Type C Mk.II turret, and the development of it from the Type C Mk.I nose turret for the Handley Page Halifax to the Mk.II for the dorsal Hudson position took only nine weeks, involving re-routing the services from the top-entry on the Halifax and adding a glazed rather than solid back to the cupola. Despite its massive egg shape on the Hudson, it was not as draggy as might be thought. I've not got figures of speed-loss for Hudsons fitted with it compared to without, but when fitted to the Halifax as a dorsal turret it 'only' cost 6 mph, according to Wallace Clarke's British Aircraft Armament Vol.1.
Here's a couple of photographs I took in 2010 when it was at an earlier stare of restoration, than the recent shots on the AWM blog with the Perspex now fitted.
As can be seen, it's a complex piece of machinery, and owes more in appearance to steam engineering than aero, but they were an effective unit.
I look forward to Jamie's report on the even rarer ventral bathtub position.