You are correct: the thin cylinder is a fuse, and the can contains a spark-gap lightning arrestor. The aerial connects to the upper fuse end, the radio to the upper switch contact, and ground to the lower switch contact.
The switch was used to 'earth' the aerial whenever the radio wasn't in use, and especially during a storm, to bypass any high voltage to ground. Note that the switch blade's only contact is to the fuse bottom end, inside the porcelain.
