Replaced a broken caged ball bearing headset with another similar type. There is a split plastic wedge type shim that seems to fit between the top bearing retainer and the steerer. It seems to center the bearing and without it there is a lot of play in the steerer. There are no instructions so I'm not sure the install is correct. Could use an illustration. Thanks for any insight.

I'd say google the headset and check for an online manual. I had to do that...If not, someone more knowledgable will be here shortly to help you.

It's most likely exactly what you're thinking it is.

It would also help to know what headset you have.

It was unmarked and came with the bike to assemble. I initially used a different fork and headset but am now going back to the original. The way I have it setup, the stem bears directly on this plastic wedge shim not the top bearing retainer so even if I tighten the top cap it doesn't have much effect on bearing adjustment although there doesn't seem to be any play in the steerer.

Some cheap headsets come with a plastic crown race, which is also split but a little bigger than the retaining ring that goes on top. If your crown race is metal and/or already installed on your fork, that little piece you're asking about goes above the top bearing.