I assume the original MkII is not like this one ; did they use a body of MkII for the new device (safety cover) ?
from BOCN :
Fuze, Percussion, Spigot, No 146
Design IDW B/65. CA.Vandervell design of allways fuze, similar in concept to the Fuze No 145 but using a ball. The mechanism consisted of a needle pellet and a detonator pellet, kept apart by creep springs. The needle pellet had a coned upper surface, and between this and the coned interior of the fuze nose was a steel ball. The two pellets were separated by a safety bar attached to a weighted tape. On firing the tape flew free and withdraw the safety bar. On landing, whichever was the uppermost of the two pellets would ride down and bring needle and detonator into contact. If the fuze landed on its side the ball moved across the coned surfaces and forced the needle pellet down on to the detonator. Broadly speaking this has been the operating principle of most allways fuzes ever since. For use with the 3in Stokes trench mortar
Mark WOLC
1 18496 of 21/1/1917 Introduction
2 22066 of 24/3/1919 Longer safety bar, shorter needle holder.
3 22066 of 24/3/1919 Built-up needle holder instead of aluminium casting
4 22066 of 24/3/1919- Modified needle holder; detonator closed by tinfoil disc.
5 22066 of 24/3/1919- Aluminium needle holder, shortened safety bar
5R A2732 of 14/3/1927 Mk 5 with a steel, instead of the-cast, cap.
5R* Not formally introduced; no record.
Mark 1 Obsolete: 24753 of 30/12/1921
Marks 2,3,4,5 Obsolete: A7676 of 21/1/1933
Marks 5R, 5R* Obsolete: C3936 of 26/9/1949 "