Bomb aimers stopwatch

My neighbour showed me an interesting piece of history yesterday. He has recently cleared his wifes uncles house after he recently died.The Uncle was apparently naval aircrew, not sure of the details though.
Anyhow in his belongings was a stop watch, but no ordinary stopwatch, this is a ‘reversing’ type.
I cant recall the maker and as yet I don’t have a photo, but will post one when I get it. He sent a description to the IWM and they have come back saying its a C.1916 bomb aimers reversing stopwatch and have sent a photo of a piece of kit which it was used in conjunction with, this kit was somehow fitted to the fuselage and certain things lined up then the aimer would set the stopwatch and bobs your uncle.
Thats as much detail as I have at the moment, but will get more over the weekend.
Has anyone heard of such equipment and whether it was any good?