.303 British MK 4 ammo, incendiary.

[917601]

I am not educated in British ammo. I saw this .303 ammo at auction and am wondering if it is collectible or rare. It is advertised only as "British MK IV 1939 dated ammo". The projectiles are silver ( cupro- nickel?) and have what appear to be a " step" impressed on the projectile. No headstamp information disclosed. See picture. Can someone identify and classify them?

 

[bdgreen]

Look here: https://sites.google.com/site/britmilammo/-303-inch/-303-inch-incendiary