thank you to all the contributors for their answers. I assume you can liken its use as similar to the mileage trip recorder in a car speedometer, substituting time instead of distance. the museum is in Haren.
as an afterthought, myself not being an aerodynamist, did the Battle have some form of windscreen/air deflector in front of the aperture for the bomb aimer ? it seems there would have been an awful draught whistling into the fus
many thanks to all the contributors for successfully resolving this ID problem.just goes to show then how big an aeroplane the Battle was. the photo conveyed to me the idea a much larger aircraft was involved.