Well...I think you should leave something as HEAVY and difficult to move--and Valuable on the property (which belongs to someone else I assume). Not so great trying to ask to remove something like that. I have seen them on properties I have detected, enjoyed them while there, and left them be. Some of those things are way out in the middle of nowhere to the point you'd have to have a crane or backhoe to get it--assuming the property owner would even let you try. You see, it's one thing to take a little handfull of metal with you in a property owner's eyes. It's another if you want to remove and keep (or re-sell) a fixture of the landscape.
You had a nice day digging. That musket sling hook is a great find.
Cheers,
Buckles