It's for a flat kit. A little pain to get the tube to stay, but nice feature.
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Always wondered what that thing I take off was for.

Mistica is great, by the way.

Did you have a tritone before? I'm on the fence about getting one, and curious if it is that much different.

I did. Moved it to my training TT bike where I put all my old upgrades. The grippy material they use on the mistica keeps your butt in place just as it was designed to do. I always moved around a bit on the tritone and the mistica also feels a bit softer. Really comfortable. Carbon rails version also light as a 205 gram feather.