I remember Bosch distributors from the 50's and 60's as having a metal dust cap under the rotor and from about 70 on as having a plastic one. I just assumed that most of them had been discarded by the time I worked on them. I currently have one of the plastic ones in my distributor.
Al
66 230sl
113-042-10-014715
904/396 blue, Ivory Tex
condition: not-as-rusty-as-before-bucket