Brian is correct that this is a British 6 Pounder (57mm) case. The designation comes from an old method of measurement where a lead ball of this diameter would weigh 6 Pounds.
III - Mark 3 Case
D - Unknown, possibly an inspector’s stamp.
CF - Round loaded with a full charge of cordite.
K - Manufacturer - Kynoch Ltd. of Witton, Birmingham, UK.
Lot 249 - Lot number for that year (1918)
B[Arrow]8 - Inspector’s stamp
1918 - Year of manufacture.
This case was used in the French-Designed 6 Pounder Hotchkiss gun. These were used as deck guns on British ships and also in early British tanks. The guns were in British service from the late 1880s to 1945. Photos of the gun and shells can ben seen at the link below:
The case should be 57mm in diameter at the open end with a total length of 307mm. There should also be a “neck” where the case diameter reduces around an inch below the open end. Brass shell cases were often cut down to be used as vases, poker stands etc.
Unfortunately this case is not “rare”, but it is still collectable.