That area of the hook was flattened and had a hole in it. The rivet holding it at that corner passed through the outside plates and through the hole in the hook arm and it would swing on that. You just need to indicate the space between the hook sides and the plates and you're all set.
Now that looks like a very old lock. The metallic look is right on the button and the rivets are nicely placed/done as well. I also love the color you chose for the background. I agree with the movement of the hook, if you added some darkness around it where it attaches this would be an absolute perfect picture.
drawn in 59 min
it looks like real metal, but technically speaking - how would that hook move?
nice job though, its really very well done
edit: i looked at the ref, it seems to twist inside i need to define that area so it looks like it goes into it rather than just attached.
I love the deep mettalic look here.