Venetian bead necklaces and Czech bead necklaces often have bead caps. Usually they are gold plated brass. Often they are stained or corroded, especially if someone dropped acid on them testing for pure gold. Variations, yes. Some are very elaborate with texture and perforations. Others are pinched into shapes. It would nice to make some sense of the caps too. But I assume it is mainly a 20th century feature.

Yes, the common barrel clasp. Variations exist. Not really noticeable unless you are paying attention.