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I recall reading that it is just a damaged bunter. The cartridge is a MkII.
The "C" indicates cordite as the propellent.
The "6" indicates the contract supplier of empty cases to Woolwich, which the Royal Laboratory then filled.
The MkII cartridge was approved in July 1893 and used until 1904 when the MkVI ball replaced it.
If you post the picture over on BOCN they will be able to give you any further information that is available.