The P200 pencils (with a few exceptions) have been plastic reservoirs since the were released in 1970. The metal reservoirs appear to have been used on the more executive style, higher-end pencils, before they too were switched to plastic.
The few exceptions for the non-plastic P200 reservoirs, are a few P320 series from the early 1970’s in Japan. Since the P320’s were exclusive to Japan, they may have used the higher end metal reservoirs on some of them, and used the plastic reservoirs for export.