This is a used Profile Design Aquarack II seatpost-mount water bottle cage/CO2 cartridge holder. For 31.6ø seatposts, no shim. It is in fair condition with some bending fatigue visible on the clamp, and deterioration of the bottle cage retainer o-rings. Two-Bottle rear mount hydration system clamps to standard round diameter bicycle seat posts. Affix two CO2 cartridges with included brackets. Bid on Gear Goat auctions with confidence: eBayer since 1999, 7000+ transactions, 100% positive feedback. For convenience we accept many forms of payment and will professionally box, insure, and ship this item for $7.00. Local pickup @ Gear Goat Xchg in Charlotte, NC always a welcome option! (c2)