These brass body clusters all have a threaded body. The cap unscrews from the base.
This large cluster has a hickey, threaded rod, and finial top to keep it closed.
Another example of a cluster with a hickey and finial top.
This early electric style cluster had many arms soldered to the cluster, but the bottom cap screws off.
Cluster with a set screw.
So when you get to a cluster and it has wires inside, there is a 99.999% chance it opens from the top.