It's a Fuze, Percussion, D.A., No. 117.
I suspect it's a very early one circa 1920s, Mk I or Mk II. It would appear that the striker cap is short, (it doesn't appear crushed) and there is no striker cover. Also, at the base of the taper there appears to be a machined groove, but it could be a gasket - the groove was present on the Mk I but not the Mk III. However, on the Mk I the shutter was placed higher up in the main body and the CE pellet was long and thin as opposed to short and fat as found on the latter marks.
Regards
TimG