I want to thank teecrooz for pointing me towards an Alien prop, and user Septic for pointing me towards the right kind of part and a whole other forum devoted to that movie. I did a little digging, and it's not exactly the part from Alien, but one like it.
These are funny things, they're generic model-making parts.. they are not caps but solid pieces of plastic with flat bottoms. This is the right size for the Barbican. Maybe even the black thing on the pommel too... not sure about that one. They fit perfectly in the bolt holes. My countersunk holes are a little smaller than the Barbican ones, maybe by less than 1mm, which accounts for less exposed metal around the part. They exactly match the head size for the M6 bolts I use.
(click on pictures for larger version if you want)
So, since they're plastic and injection molded they are not all perfect looking. Some have flashing or rough lines, which seems normal. And get a load of this. They are clear. *Some do appear to be available in white, which is a possibility too!*
Even the similar part on Alien is clear originally. Maybe they got a layer of primer before the paint, because it does kind-of look white underneath the black paint. The alien prop is coated in that mystery green paint. I painted these with some Rustoleum flat black and shined them up with a paper towel. Looks like the Barbican used a satin paint.
As usual with the way the hobby is, message me for the model number and specifics. I don't care if my inbox blows up, I just want to protect this part from scalpers, parts flippers, etc. This is not a rare item and I'd like to keep it that way.
I'm going to put these in silicone and pour black onyx casts for myself. They're tiny, so I will have a lot of extras and I can make them available for a hell of a lot cheaper than I paid to get these here) more on that in the proper forum section.
Neat huh? I can't get over that they're flat, it's so funny to me and I can't explain why