Clancy,
If you are talking about the top large hole then, it was used to hold the spring, firing pin, the spoon, and the ignition cap. It was develped this wa so that all of the parts of the ignitin it is for the post that holds the spring and the firing pin.
However, if you are talking about the small hole running through both sides, actually both holes run through, it is a securing hole from what I was told in "Booby-Trap & Trip Wires" School and it was one of the ways to disarm the grenade that has been used in the Booby-trap configuration.
I am not sure when the transformation took place in the assembly of of the fuze butI have seen them in both the TM's, FM's, and in books that I have about WWII, Korea, and The Vietnam War. This showed that almost all of them had the housing that had the the holes for some reason.
It was at this School that the instructors showed the class this little technique/trick to prevent it from going off. You would use a piece of of heavy gauge wire or by using a small finishing nail to insert it through the small hole and then be very careful that the Booby-Trap is not resting on top of a another Booby-Trap such as a pressure release devices, M5 for example or any other Countys version of a pressure release trap.
This is what I was taught so please bare with me on this idea to use. Also, would appreciate any comments?