Thanks, the rear part of the brass sheath looks like that (where it's 'doubled up'), but I don't think it was meant to be there - this is certainly not a 'flippable' seatpost, so I think that's not the same model (I think the modern P3 has a flippable seatpost?).
Googling "cervelo seatpost shim" lead me to: https://www.sigmasports.com/item/Cervelo/Replacement-Shim-for-Flippable-Aero-Seatpins/48A