The photos below are of three pairs of factory 906 camshafts. Some with just the mechanical tachometer drive and others with both, tach-drive and MFI Pump drive. By the way, no re-grinds here, these are factory parts.

About the "Special Fine-Threaded Nuts and Convex Washers" used in these camshafts: Here are some photo links showing a dial-caliper to the 906/RSR Special Nuts and Washers. 32mm I.D. Special Convex Washers https://pbase.com/9146gt/image/45499782 --- 30mm O.D. Camshaft Shoulder https://pbase.com/9146gt/image/45499784 --- 27mm I.D. Special Nut https://pbase.com/9146gt/image/45499783 --- More about these Special Nuts. Don't pay too much attention to the O.D. of these nuts. Here is why. I have seen the special Nuts O.D. made in two sizes (don't ask me why, I have yet to figure this out!). They were made available with a 41mm O.D. https://pbase.com/9146gt/image/39886397 and also at 46mm https://pbase.com/9146gt/image/45499781 - The I.D. of these 2 different nuts sport the fine-thread pitch with the same I.D. diameter so they both work on the 906 and 911 RSR camshafts. Hope this helps some of you.