Interesting. They may have done training in the field or it might have been a procedure to keep a round in the chamber during down times, maybe to keep mud out of the action or something? Not very logical, but who knows?
It does seem that it has a primer. It could have been an inert primer. There is the school of thought that dry firing will damage the firing pin, and in some cases this is correct. But, most if not all military arms will withstand dry firing with no problems. They make training rounds now days out of plastic with a silicone or spring loaded cap to give the firing pin something to hit.
It is not a shootable round. The flutes would allow for unwanted expansion of the case causing potentially catastrophic damage to the gun and the shooter.
I don't know what the headstamp means...if you feel like going into the world of cartridge identification/collecting...here's a place to begin...
http://www.surplusrifleforum.com/viewto ... 134&t=5807
Cool!
What else have you got from Flanders Fields?