Only this cap was found, and it appears to be made out of some plastic material as it is flexible at the open end. Perhaps that is why the metal ring was needed, to give it some mechanical strength at the open end.
Looking at the close-up below, it appears to have a brushed quality with the lines.
Here is a complete plastic BP in olive green.
A closer inspection of the cap itself shows the same striations in the plastic and the reinforcing gold ring as the black cap above.
Another example of a plastic BP, this time in brown.
