1901 British Shell Case with Unusual Primer

[garys39]

Need some help here, bought 2x of these shell cases today, I can't find any reference though to what the unusual primers are or why they are this shape ?
Dated 1901, made by Vickers Son & Maxim, case measures 200mm long and 84mm approx calibre.

[garys39]

A couple more photos.

[klarkon]

It is shortened case for QF 12-pounder 12 cwt gun.
The case has an Mk.V or Mk.VI adapter.