I think this is correct:
The automatic transmission filler tube, when original, is CAD plate with a phosphate wash; this is the "gold" color CAD.
Some restorations will re-plate this with zinc, and then a chromate wash, giving it a gold color plate, but different gold. Experienced Concours judges will be able to tell a zinc-gold from a CAD gold.
The replacement part from MB today is painted a chassis black.
The filler cap on the dipstick, when original, was a CAD plate with no phosphate wash. That means it is silver in color.
Some restorations will re-plate this in zinc; some in natural zinc which is silver in color, (but a different color then CAD) others will "throw it in with the rest of the engine bay parts" and give it a gold color, again, usually zinc with a chromate wash.
If I am not mistaken, either current replacements or later model W113's (maybe 1970-1971) had a black PLASTIC top.
Having been through this very issue several times and discussed it several times with experienced Concours judges, I'm fairly certain that what I state here is accurate. I also lived it: my original tube was replated in zinc during the restoration; replaced with a new black one when the original was cracked (use 2 wrenches!!!); finally replated in gold CAD. The dipstick top had been incorrectly replated in gold zinc color during the restoration. This past summer one of our members and friends, Tom Rose, was gracious enough to call me in advance of some real CAD plating and I finally had my dipstick filler top replated in correct silver color CAD. Because my otherwise "pretty nice" engine bay had these little details incorrect they were always the subject of discussion during judging.
Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America