BMC changed to a normal seat post clamp in 2012. Previously, they used what they called a Streampost which had an elastomeric expansion plug at the end of the seat post. Though there's no real benefit and it weighs more, it works fine as long as you don't lube it and keep it adjusted properly. Some experienced seat post squeaking with this arrangement, the fix being putting a piece of thin plastic tape around the back of the seatpost where it exits the seat tube.
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