Oh so interesting facts about Brompton seat posts.
1. One of the reasons for using the Pentaclip is to allow removal of the post.
The bottom end of the post is flared to prevent it popping out when you raise it.
If the post needs to be changed, removal of the Pentaclip allows the post to drop out at the bottom of the frame.
2. There is a black plastic shim around the frame seat tube near the quick release.
This means the post can never stick, grip tends to be good due to the gasket effect, and the post is most unlikely to be scratched by the clamp or by moving it up and down.
Obviously good benefits on a Brommie where the seat is up and down all the time.