A while ago I took apart my grandpa's really old radio receiver with vacuum tubes. I recognize everything except for this one strange component. It seems to have some sort of heat sink on the outside and it has a cylinder-like shape on the inside. On one of the sides, it says 6846446 and + and Zenith and 212-23. The + seems to tell me that it is some sort of polarized component but I have no idea what it could be. Here is a picture:
That's a selenium rectifier. Google "selenium rectifier" and there's loads of info, photos. They've been obsolete since the 1970s. Decades ago, they were cheaper than vacuum tube diodes, and semiconductor diodes couldn't handle higher voltages and currents. The parallel plates are indeed for cooling.