These things are so hard to find, I tried so hard I ended up with two of them.
Reducing crown race for Cane Creek XXII Headset (as used on Trek Session 88) to allow the use of a straight 1 1/8 steerer with the tapered (E2) or 1 1/2 head tube.
It's Cane Creek Part No HD1406. There aren't any in the UK, not that I could find anyway, so I had to get this shipped from the States.
They're £10 if you can get them here, I paid £30 to get it from the States, £15 for posted sounds fair to me.
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