Hi Chris,
What you have there is a No 68 anti-tank rifle grenade. If you are familiar with the grenade launcher for the No 36 Mills bomb with the grenade discharge plate screwed onto the bottom, well, the No 68 is fired from the same cup discharger. You will find a photo of your grenade at "The Grenade Recognition Manual" located at
http://members.shaw.ca/dwlynn/tgrm.htm . I have a small collection of grenades and don't have the No 68 yet and would really like one if one ever became available (hint...) Would I be able to get some detailed photo's?
Steve