This photo gallery shows some samples of similar 917 Blau fuel caps, but these are the more common and less desirable as they are from a later manufacturing period. Although some of these Blau caps were made with the early Aluminum tops, they have a 'steel centers' identifying them as NOT the earlier (917K) part. The later production caps also have cogged edges and incorporate the 11 raised clock marks. The steel centers https://pbase.com/9146gt/image/25121890 fail the magnet test as the magnet WILL STICK to the steel center.

You can find these steel center Blau caps on eBay between $50 and $100, not much more. The early Blau caps with both Aluminum 'Tops' and 'Bottom Centers'? Well, that's a completly different story--worth hundreds of dollars more and mostly found in original race cars and high-end Porsche race car projects.