20 watt dummy load. This device is inserted inline with one or more speakers in a multi speaker cabinet. The purpose is to be able to turn off those speakers for lower volume from the amp, while retaining the proper impedance match to the amplifier.
These are not attenuators with volume controls. They are simple on/off devices that are designed with something like a Tweed Bassman or Super Reverb in mind – where you have four 8-ohm speakers and you can put one silencer inline with each speaker you want to be able to turn off.