Tom,
Beware of very early 230ies! There are a lot of differences in comparison to ordinary 230ies (64 -). All depends on the serial no. The pic in the DB publication (dealer's brochure) is definitely a very early car or even pre-series. So, this pic is only of very limited use for a, let's say, 65 SL.
I bought two new caps from DaimlerChrysler a couple of years ago, the same Michael refers to. They're definitely yellow zinc.
The old original one I had was also yellow (in the hidden corners) behind rusty overall appearance. I got it replated.
Very difficult about the throttle linkage rod across the valve cover. In most cars (late 1963 and so on) it was yellow cad (i.e. zinc). Maybe very early 230ies had black ones - very hard to find out. I have seen a few very early cars, however no clear reference. Most likely - all yellow.
Best,
Achim
(1971 230 (without SL) W114
1990 Opel Vectra A)