Might not be quite what you were thinking of but . . .
This rig was photographed in one of Short Brothers factories – I assume either for testing hydraulics etc or to demonstrate to some of the new hands how it works off the ‘plane before letting them have a go fitting it.
At that time they were building Sunderland IIIs so I think it is an FN11 front turret (the back had a four gun FN4a). Do correct me if I’m wrong.
Allan
Very usefull info there,thanks for posting 😎