I use Purple Power mixed pretty strong, forget about them a day or so, then blast off with water. Pressure washer is best, but I used the spray of my garden hose. It's not kind to the labels, so carefully remove if that matters. I think those can be reproduced, but don't have the source available. There are plastic polishes to give buckles the royal treatment.
Hung out to dry:
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* All these are long gone. Sold to Jerry Brown years ago