Whilst Roger gave you the meaning of the "VII", i.e. a Mark VII ball round, given the circumstances and location of your finds I suggest that in fact it is the remains of a grenade discharging cartridge.
In WWI these were made from reject ball cases and so carried a ball headstamp, usually without the original ball marking being struck out.
The sequence of the grenade discharger cartridges in WWI is quite complex, and once removed from the original packaging they are difficult to identify.
[FONT=&]The first of the rodded grenades was introduced in 1915 and the discharger cartridge for it was approved initially as “Cartridge S.A. Blank .303 inch Short Rifle Grenade Mark I”, but on introduction of other types the title was changed to “Cartridge S.A. .303 inch Rifle Grenade 35 grains Cordite Size 3 Mark I”. This used an uncrimped ball case with the above charge with a tuft of guncotton at either end of the cordite and closed with a cardboard disc or cup sealed with shellac. The original ball headstamp was supposed to be cancelled and overstamped with “I”, but no such examples have been found.[/FONT]
[FONT=&]The next type introduced in 1916 was a short term measure whilst a more suitable round was developed and was the “Cartridge S.A. .303 inch Rifle Grenade 37 grains Cordite MDT Size 5-2 Mark I”. This was simply a Ball Mark VII cartridge without the bullet and with the neck filled with tallow. It was provisionally approved for use with all rodded grenades and only seems to have been in use for a short time. [/FONT] [FONT=&]
Following this came the “Cartridge S.A. .303 inch Rifle Grenade 43 grains Cordite MD Size 4 Mark I” designed for use with the No.22 grenade with 15 inch rod. It was, like the others, made from uncrimped ball cases closed with a paper disc and sealed with shellac. It was blackened all over. Later in July 1917 a second version of this, the “Cartridge S.A. .303 inch Rifle Grenade 43 grains Cordite MD Size 4 Mark II” was introduced for the No.23 Grenade with six inch rod. It was identical in appearance to the previous Mark except that the neck was sealed with a cardboard cup and both Marks had the usual tuft of guncotton at either end of the charge. There seems to be no difference between the specification of the two Marks although it has been stated by others that the Mark II actually used Cordite Size 3.[/FONT] [FONT=&]
The Mark II remained in service until the end of the .service life of the .303 inch cartridge, the nomenclature changing to “H Mark II” in 1927 and eventually to “Cartridge S.A. Line Throwing .303 inch Cordite H Mark 2” in 1948 after it became obsolete for Land service but continued in use by the Royal Navy. [/FONT] [FONT=&]
The cup discharger for the Mills grenade was introduced in 1917 and a new cartridge followed in August, the “Cartridge S.A. 303 inch Rifle Grenade 30 grains Ballistite Mark I”. As before, early production utilised uncrimped ball cases with the top half blackened for identification. Shortly after WWI the headstamp was changed to include the numeral “1” and from 1928 it became the “Cartridge S.A. Rifle Grenade .303 inch Ballistite H Mark Iz”. In that form it remained in service until the end of the Second World War.[/FONT]
The above details are from my book "Headstamp Guide - .303 inch British Service Ammunition".
Your case is too corroded to tell which type it is or whether it was ever blackened. Also, I think the headstamp is "K-18 VII" and not KN. K is Kynoch, Birmingham although I am sure you know that.
Attached photo shows the Short Rifle Grenade Mark I, Rifle Grenade 30 grains Ballistite Mark I, New Zealand H Mark Iz (WW2) and Rifle Grenade 43 grains Cordite MD Size 4 1/4 Mark II. The latter
was introduced for the No.23 grenade with a six inch rod.
Regards
TonyE
