What are the ratings of vintage Selenium Rectifier B20/16-2,4?
You're unlikely to find specs on the selenium rectifier.  However, you can probably get a pretty good idea from context.  You must know something about the device this is in.

Chances are, a 1N4004 can be used as a replacement, although you might have to put a resistor in series.  The old selenium rectifiers had significant resistance, and sometimes the circuit relied on that, like to limit inrush.

These rectifiers were usually limited to the power supply, so taking a measurement of the power supply voltage or otherwise inferring it would be useful.