Haven't a clue! however the WD and the broad arrow definitely means it's UK military issue. The fact that the inner screws into the outer means there was some reason why a simple push fit wouldn't do and presumably either a need to keep the "spoon" clean/ undamaged between uses or perhaps after use a need to seal it away to avoid contaminating other things. (Medical, possibly) Brass also seems a little unusual. Softer that steel, won't rust but will tarnish - maybe there was a need to make sure it couldn't cause a spark?