Thanks Alan – I have the drawing of the type B in the British armaments book and it shows the large mirror but no details or codes – much appreciated.

The book says that the type B periscopic sight was not widely used..

Also 1941 date suggests it may have been from the initial early B mk1 first fittings ? or Manchester ?

Nice picture here:
http://www.arrse.co.uk/community/threads/bomber-command-memorial-a-german-perspective.176044/page-25

and some disparaging comments in this thread post 19 onwards.
http://forum.keypublishing.com/showthread.php?49716-Aircraft-turrets-(any)