[Graham Boak]

It was not retractable at all. Initial Blenheim Mk.IVs did not have it. I don't know the dates of introduction, but I don't think there were any in the Battle of France timescale. Later ones had the slim transparent blister with one rearward-firing gun, as seen at the same time on some Beauforts. Later still this was replaced by a boxy metal structure with two guns.

Re being semi-retractable. This may be a confusion with the early dorsal turret, which was semi-retractable.