Vintage 5" Heavy Duty Marine Swivel Lobster Landyard Connector 9oz Steel
We have many of these.
I feel like they may not have been used.. however, they are a little rusty, some of them.
The one I weighed was about 9 ounces, which is quite heavy!
And near the top of the hook it was about 0.5 inches thick, (narrowing down to around 0.47 inches at the very top).
Clasp works good and moves freely.