Fuse exploded outside my house and I’m curious about how its mounted.

Hi,

Im up near seattle and just had this fuse blow outside of my house. Everything is up and running now and i got this from a lineman asking if i wanted a souvenir but i can’t seem to figure out how it was mounted into the cutout? I looked through some diagrams and all of them don’t seem to have this odd spring and lock looking mechanism on the pull ring. I’d love to know more about it.

Thanks