This came from ex-military disposal sales in the 1950's and was always thought to be some type of recoil mechanism for a field piece.
Only after having a recent look at the thing i am convinced that this was not the case !.
Resembling a large bike pump, the body of the item is 22mm diameter and made of steel with a point at one end...the knurled handle/grip is made of brass.
It is 820 mm in length and can extend to 1280 mm by twisting the knurled grips, sliding it out and twisting the grip to lock it at any any point.
The grip is marked O.S.905-A with a small broad arrow which looks to be made by 3 chisel marks.
Cheers .. Kenny.