Many stem manufacturers started to use those top plastic plugs in the late 80's and into the 90s. A lot of them were not used by the owners and eventually got lost in junk drawers or they just fell off the bike during rides. It was really something that was not needed for the function of the stems but just an opportunity the company to put their logos on the stem in place of engravings on the stem. I could imagine that finding specific plugs for specific model stems would be quite hard to find these days. Maybe the last resort is to have one 3D printed if there might he any way to aquire a model of the plug.