Actually I found it in a hardware store believe it not.<br>It was lablled 'Survival whistle' and touted a waterproof compartment for pills.<br>I suppose the size is right for GTN (heart pills), but is also great for chlorine water purification tablets. I think iodine would react rather nastily with the anodised aluminium.<br><br>If you have trouble locating on I can get one and send it to you (via the lunch box / trangia box shipment I'll be making to schwert soon.... sigh, still waiting for the stock to arrive... )<br><br>