That's really great, congratulations! I wish it was mine!

I've seen others inerted like this one, but from what I've seen it's less common than by cutting into the detonator tube area, which is easier. I've heard that this was sometimes done with a machine, which would explain why it looks so good. I don't know how you could do this by hand without ruining it. If this was taken as a souvenir from the factory that produced it, they could even have used the same machine that did the original crimp. I wonder if anyone else on this site can shed some light on this? I don't know that much about it.
After I posted the comments above, I looked again at the pics, and I see tool marks from it being opened, so I think it was actually done by hand by someone who was very careful and did good work. The other examples I've seen that were done by hand looked terrible.
There are also rack marks on the handle from it being inside a grenade case. This grenade really is a great find, I think.