I agree with oooldman- looks to be an internal combustion engine's rocker arm. If you could clean it up with a wire brush I think that's what you'll find out it is.
I'd originally thought of the same thing, but wondered if maybe not, due to the hole being "too round" rather than elongated to accomodate the pivoting they have to do...and the hole isn't quite in the deepest part of the "socket" as would think it would be... But I agree it looks a lot like one... And vote for a cleaning!
This may sound funny but if it steel/ iron (check with magnet) you can soak it in automatic transmission fluid- at least a week. I use this method on old rusted files and other old tools and you will be surprised how clean the item comes out- NO RUST. Not real fast but it works. HH