If your bike has a Trek Knock Block headset system, you will need this adaptor in order to fit your Shotgun Pro.

Fitting the Shotgun Pro on a Trek bike with a Knock Block requires a minimum of 30mm clearance under your stem (from headset to the bottom of the stem).

The Trek Knock Block adapter replaces one of your 10mm spacers, the Shotgun Pro spacer (provided with your Pro seat) replaces another of your 10mm spacers, one of your own 10mm spacers remains directly under your stem. 

If your stem clearance is less than 30mm you may need to create more space under your stem to fit the Shotgun Pro.