I confirmed with my Tern contacts that the newer Terns do not have the shim. News to me! If the post and the inside of the seat tube are *verifiably* grease-free, nad you have the clamp adjusted so it's moderately hard to close, the seatpost should not slip. When my Tern was new, I had some slipping initially that went away after a couple weeks... I think the surfaces needed to wear in a bit. That was a shim-based system. You might consider wiping the post and the inside of the tube with acetone to degrease the interface. I would lubricate the cam on the seatpost with a good high-pressure grease, being careful not to get any on the seatpost-seat tube interface.