The Porsche early center-lube racing camshafts use a pair of special fine-threaded nuts and 'convex' washers (concave). Here are some photo links showing some dial-caliper measurements of the 906/RSR Camshaft Special Nuts and Convex Washers.

Convex Washers: 32mm I.D. https://pbase.com/9146gt/image/45499782

Camshaft Shoulder: 30mm O.D. https://pbase.com/9146gt/image/45499784

Fine-Threaded Nuts: 27mm I.D. https://pbase.com/9146gt/image/45499783

Don't pay too much attention to the O.D. of these fine-threaded nuts. Here is why. I have seen the special nuts O.D. made in two sizes. Why? I don't know. They were made available in both, a 41mm O.D. https://pbase.com/9146gt/image/39886397 and also in a 46mm O.D. https://pbase.com/9146gt/image/45499781 - The I.D. of these 2 fine-threaded pitch nuts is exactly the same so they work on both the 906 and 911 RSR center-lube racing camshafts. Ask Jerry Woods, I did. Hope you find this information helpful.