Kirk King Cobra Action Head as shown. I think this originally cost me £400 but it was an amazing bit of kit that made it possible to perfectly balance a 500mm/600mm lens on a tripod so that it could pan and tilt with minimal effort. Also useful for smaller lenses if you want the freedom to move them about without having to loosen any knobs or risk the camera falling down. Has an Arca Swiss style clamp mount for the lens, and screw fitting to go on the tripod (I used a Manfrotto). Both horizontal and vertical lens movement can be locked or dampened by turning the large handles. A seriously great bit of kit but I don't have the lenses any more!
eBay