I am as close to being 100% certain, without having tried it myself, that the PO2500 straps and bracelets will fit the SM300 perfectly. In fact, I think I've seen a couple threads about that on WAtchuseek.
The metal PO 2201.50.00 20mm bracelet is the 1580/952:
http://www.ofrei.com/page1280.html
Mark
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