Here are the pictures of the British BIV trade pattern incendiaries as promised.
Left is a 1940 dated example of the round you have which is a B Mark IVz, centre is the other type, a cordite loaded B Mark IV, and right is a normal B Mark IV. Note the slight variation in ogive and position of the bullet shoulder.
Headstamps are written on the cases but are:
This is the 1940 Kynoch drawing showing the CN covered fluted plug.
The following is the entry in the British military “Regulations for Army Ordnance Services, Part 7, Pamphlet No. 11, Small Arms Ammunition” which shows the B Mark IV*z designation for trade pattern incendiaries.