From the manufacturer:
This pair of C-posts are specially shaped to fill the pillars on the classic Mk3-on Mini, until the introduction of the seatbelt mountings into this area. Included in the kit is a strip of spare material to cover the exposed body between the rear window rubber and the parcel shelf.
As with the originals these need to be bent into position so that they hold their shape partly under their own tension, with the loose flap of material at the bottom being glued down onto the shelf underneath the rear parcel shelf.
The C-posts and spare material are included in the standard headlining kit HL3012.
See CP3012 for additional colors. Actual colors may vary from those shown.