During a recent trip to "Wings" I came across this AA rangefinder in a rather cluttered corner of the museum. I'd be interested to hear anything you know about the use of this piece of wonderful equipment. So far I have found this article on the theory & use of rangefinders as they relate to the US Navy: The Gunnery Pocket Book - Part 3 I would like to find out more about how they were used by HAA in WW2, but I'm sure that any information you have on this subject would be very interesting.
You might find this of some interest, Barr and Stroud being the principal manufacturers of height/range finders for AA. Barr and Stroud Rangefinders - The Dreadnought Project
Sorry, I don't think this is a rangefinder as the optics are not right. The two lenses need to be as far apart as possible for best accuracy. Judging by the photos here: 27th December 1942: The life of an ATS ‘Ack Ack’ Girl ...this appears to be an "Identification Telescope".
Telescope, Identification, AA, MkIII (Rangefinder) , case & tripod stand TELESCOPE, IDENTIFICATION, AA, MKIII (RANGEFINDER) , CASE & TRIPOD STAND TD ATS girls using an identification telescope Manufactured by Ross & Co