I've recently acquired a retro steel bike which needs some work. Has anyone ever seen a freewheel with this type of locking system? I haven't been able to find any tool which seems like it would be designed to open it. Note the splines are opposite from a normal lockring (ie: small gaps on the outside). I've heard some older freewheel bikes have 'unremovable' freewheels- is this perhaps one of them? There are no identifying marks or brand logos.