The tops are very soft pewter or other alloy, and mark very easily indeed! It can be done as Jez says, but you do absolutely need the right tools! I believe they used to use a bench-mounted 'cup' (for want of a better term!), slotted to receive the lever and nozzle, then special tongs to turn the split ring. (Better to have slight cloudiness or old contents inside, than a wrecked syphon cap, I think!)