1971 Philco color Tube Hybrid

[Electronic M]

Looks like a silicone diode. If it's one of the power supply rectifier diodes that are fed 60Hz from the transformer then you really only need to know 2 specifications of the original to replace it with a modern part. Those specifications are PIV (peak inverse voltage) and current rating, and make sure the new part has those numbers as high or higher.
For power supply rectifiers I stock 2 types that can handle everything....1N4007 (1A, 1KV PIV) and a 1N51?? that's rated for 3A 1KV PIV....The higher current part is pretty much never necessary, but anything over .75A I try to use it for so I'm not running the 1A part too close to it's rating.

In convergence and detector diode applications switching time, junction drop, and capacitance become more important.

Boost and focus/HV applications are pretty much the same as B+ rects but with faster switching time requirements.