McVicker's had the right of it, in that he scored the whole deal. I figured that many of you would figure out "spike bayonet for the No. 4 rifle." That's pretty hard to do a whatzis with that isn't essentially impossible to figure out, by only showing 1mm square black... hmmmm.... [shakes head]
Anyway - it is indeed a bayonet for an Enfield No. 4 stuck on the postwar entrenching tool, intended to give your average Brit grunt something to poke the ground with to find mines with other than his foot.

And, just because I know you want it -
a picture of it separated into the fiddly bits.
Heh. The lurker swoops down from the rafter for some easy pickin's...