Motorola 50M25Z5 Zener Diode, 25 V, 50 W, Gold TO-3, 1N2821B Equivalent

Fifty watts in a zener diode is a serious amount of heat to put through one junction, and the package tells you the designers knew it: a TO-3, the same outline used for power transistors, bolted flat to a heatsink so the metal can carry the heat away. A half watt glass zener of the same 25 V would be a hundredth of the device.

Parts like this did the job a modern regulator chip does, and did it in places where a chip still will not go. Clamping a supply rail against a transient, holding a reference across a high current bus, protecting a load from an overvoltage that would otherwise reach it: all of them want something that can absorb real energy rather than something clever that gives up. The rating here is 25 V nominal at plus or minus five percent, with a zener impedance around 2.7 ohms.

The gold finish on the package and terminals is not decoration. It is the mark of high reliability construction, chosen for corrosion resistance and stable contact over decades, and it is why this looks the way it does after so long in storage. The part cross references to 1N2821B and is documented as an alternate under NSN 5961-00-881-1618, which is the route in if you are working from a military parts list. Never mounted, untested.