After cutting the ISP, PT uses a firm rubber top spacer - this takes out any non flatness from the cut, protects the carbon and adds a tiny bit of damping.
Available in 2mm or 4mm heights. Photos show the 2mm version
Slots into ISP and protects the carbon from the hard CNC Aluminium edge in the seat clamp. Fits 2021+ TCR isp.
Remember the PT cutting guide and this part are no substitute for carelessness with a saw! It is up to you to cut the ISP correctly using care and attention :)