Here's a photo looking under the umbrella and into the threaded pinhole. Note there are two small wire size holes (only one shown in photo) that enter under the umbrella on opposite sides, connecting to an internal fuse. This provides a "dryspot" from moisture and rain to ensure the line doesn't short out. Its primary function however appears to be designed to provide a circuit breaker in the event the system gets overloaded via lightning strikes or power surges. The top umbrella can be readily unscrewed to reveal a holder for a fuse to set inside the slot. Possibly used on overhead lighting systems along Germany's old railways. Cool piece!

For another photo and details [id=195519498].