Bill: Seems from this thread it may be possible one
gentleman has one thing you need; another has the other.
Take a look at this pic I think is what you would like
to reconstruct.
I saw Ross (guzzul), something like that, has the
bulb holder, the black plastic you describe. I notice
he shows the slightly rusty piece of metal. This is a
clip that holds the black plastic piece you're missing
into the underside of the light bar. You'll need them
both.
Then the two-paddled part shown by mrv.. something
gentleman, (please take no offense folks), is the actual
bulb with wire attachment. That bulb slides into the
black plastic Tylenol-type container you describe.
Clear as mud, huh?
Anyway, I think you need each of these pieces.
Attached is the pic I hope will help.
2wrench
https://i169.photobucket.com/albums/u222/2wrench/PlinthPic.jpg