You're right to suspect it's related to heat dissipation. That top triangular piece is almost certainly a heat sink.Here's why: Rectifiers Generate Heat

Rectifiers, especially older ones using technologies like vacuum tubes or selenium stacks, are notoriously inefficient. A significant portion of the AC power they convert to DC is lost as heat.

Heat Must Be Managed

Excessive heat can damage or destroy the rectifier components. Effective heat dissipation is crucial for reliable operation and longevity.

Shape and Material

The triangular shape maximizes surface area. The fact that it's metallic suggests a good heat conductor. The large surface area allows the heat to radiate and convect away from the heat source.

Placement

The top placement takes advantage of convection. Hot air rises, so locating the heat sink at the top helps to naturally draw cool air from below and vent the hot air.*

Connection to Components

The heat sink would be directly mounted to (or in thermal contact with) the components generating the most heat (i.e. selenium rectifiers, diodes, etc.)

Confirmation from Your ImagesNameplate Data

You stated you provided the nameplate data, however I do not see it in the pictures.

In summary, the top triangular metallic piece on the rectifier is almost certainly a heat sink, designed to dissipate heat generated during the AC-to-DC conversion process. Flag for review