Tom - this page from PET may serve to illustrate the difference between the two 356 Carrera parts:
http://farm5.static.flickr.com/4011/...d80cb379_b.jpg
Item 3 is described as a 'clamp' (part no. E 692.107.363.00 whereas item 9 is described as a 'retainer' (part no. E 692.107.357.00).
Incidentally the lines are listed as 14.7mm diameter.
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