Soak in a 12% solution of muriatic acid and water
for 30-40 minutes. Just be VERY careful to follow mixing and handling instructions. Alternatively (and safer), undiluted white vinegar, although it'll take longer and may require more than one application. Whichever, help it with a wire brush.Those are a couple of very impressive toilets, real bronze "thrones" built to last at least 100 years. So although W-C no longer makes the Imperial model (it's been replaced by the Skipper and Skipper II) but they'll continue to support it with parts forever. Comparable new is about $1000...even used, they're worth $250-$300 if they're in decent shape. So even though you only want to use one, for pete's sake don't just toss out the other one!