Does the mousing work by pressing the button and sliding the sleeve down? Whatever it is, it is designed to be under variable tension, or it wouldn't be moused like that. With a single photo and no size comparison it is hard to say. Maybe a form of locking sister hooks for a runner or baby stay. Is the fixed eye for a line or a pin? Any makers mark?
How long is it? Does it swivel? Send picture of the other sides as well as a close up of the pin. Ie it looks like part of the shaft is milled/cast flat with a couple of spot welds on it? The hook is broken, I suspect that you do not have the piece?
Snack eyes come in many shapes.
All of the specific examples given by the other responders could also be genericly described as scratch eyes. It depends on what it is connection together.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making any potentially dangerous or financial decision, always employ and consult appropriate professionals. Your circumstances or experience may be different.