Hello JP,
Yes, mostly brass. Your platter is correct, aluminum does not take chrome well. The lower trim is aluminum and it is not chrome plated. This aluminum lower trim on your car undergoes a different type of process sometimes called "brite dipping" which is not a plating process. It is very hard to find someone who can do this. The lighting industry uses the same process for achieving a mirror like finish on aluminum reflectors.
The door handles, trunk star, mirror base are a Zinc casting. The bumpers are steel of coarse. The "base of the windshield" trim on the sedans of the era, started off being chrome over brass and then was changed to "brite dipped" aluminum in latter years (not sure the W113 did?).
As Greg posted, brass is a favorite for low production parts. It is easy to chrome or re-chrome but is costly and a little difficult to repair if bent or damaged.
Very low production model Mercedes like the ponton cabriolets had all brass trim. Each car had all it's brass trim fitted to the cars in natural brass. After fitting the brass trim was removed and chrome plated, then installed on the cars!
Joe Alexander
Blacklick, Ohio
