I think I've tracked down the bolt size that I require by plugging part numbers into Google. In the S2 parts book I have, there were two part numbers associated with the bolts that fix the cam carriers to the cylinder head: 16044021 and 16044321. Interestingly it seems that these part numbers, besides being used by Fiat, Alfa Romeo and Lancia, are also valid for Ferrari...
Part number 16044021 is an M8 x 1.25 bolt with a length of 40mm. Part number 16044321 is an M8 x 1.25 bolt with a length of 55mm. The difference of 15mm corresponds to the depth of the additional bracket and spacer that the bolt has to pass through, as well as the cam carrier and thread depth.
So while the S2 parts books doesn't show the bracket and spacer it does appear to acknowledge that there are two different bolt lengths in use. My S2 parts book showed exactly the same image as that provided by rossocorsa, so I think that's as good as it's going to get. And if you were trying to track down part numbers for the bracket and/or spacer, I think you're out of luck...