While trolling the Net for flat whistle info, I found a page on getting the weight of the ACR whistle (with lanyard) down from it's already svelte 7 grams down to 3 grams! (and getting the bulk of this already tiny whistle down, too!)

http://www.backpackinglight.com/cgi-bin/...thread_id=33706
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A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)