Original cars and factory literature are your best sources of accurate information about originality.
Not precisely, I'm afraid. I've seen conflicting and or incorrect information in factory literature, and factory authorized literature. At the moment, I've been working on a W113 Buyer's Guide (summary article, not a book) that will be published in The Star later this year. One of the sidebars is by the late Frank Mallory, highly respected collector/author, regarding running changes to the W113 from inception to end of production. Here is what Frank said:
This list was compiled from manuals, parts books and observation of original cars. The generally excellent factory literature is not very reliable in this case; for example, Edition A of the 250 SL parts book shows one-piece wheel covers that were not adopted until a year after publication.Further, the list had a few errors on it picked up by Peter Lesler, also highly respected and recognized. Two other members of our group here have been
Shanghaied volunteered to read the draft and I'm certain, quite certain, that they will pick up some additional errors--errors that might have been in factory literature but have proven to be wrong like the one case that Frank pointed out.
Bottom line is, nobody and nothing is perfect. We strive for perfection however, and one of the purposes of the collaborative Tech Manual, (and indeed, the forums) is to share our findings.