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Tread Lite Gear says:
Rucksack strap bottle pocket in black Dyneema X Grid with polyester mesh front. Will take bottles from 500ml up to 700ml including the vast majority of sport bottles. Rear has an elasticated strap loop and an anchor loop complete with slik clip to attach on your pack strap. Top collar has bungee cord to compress. To fit, remove the pack strap from the asjuster buckle, slide the pocket on and re-thread the pack strap.