Stevens & Sons Glasgow and London , see British Whistle Manufacturer History , The whistle was Probably made around 1850’s and designed by Richard Porteous. It is marked a Model # 36 on the body.
( August 5, 1802 – Jan. 3, 1881)
Closed position 
It is a brilliant design since while other slide whistles of the period 1840’s and 1850’s had a sliding rod which could be moved in and out inside to determin the pitch , this design has the cap top around the body and the sliding part is out side the whistle’s body.
Open position & back side
The Three parts
Certainly a very innovative tricky design, of a Slide whistle, unlike most slide whistles it has a smaller register within a range of a 5th, not a full octave. C – G notes. the picture above shows its unassembled 3 parts.
The shorter part has a fillet and longer one a flare outward , so once the longer part (middle of picture) slides into the shorter part which is just a bit wider and comes to a stop at the flare, then the loop – knop top screws on top and the and by pulling the knop the slide works making the whistle shorter or longer.
The top part is cast and milled and has a screw thread on the inner part, all made of one piece.
All in all very rare and extremely well made whistle.
Made of German silver.
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I found an old whistle on the coast of Port Victoria, South Australia many years ago seems to be made of lead – I believe there are a number of shipwrecks off the coast which went down around 1870 – I’m thinking it may be a life jacket whistle – I can send a photo if anyone of you are interested in seeing it – as I haven’t been able to get any answers from anyone here in South Australia.