Early DV-6/Type 20?
DC Det/Amp Triode
1920

The Moorhead/DeForest Type 20 was advertised for one month only (May 1920) and not pictured. The exact form of this tube has not been determined, but one theory holds that the tube shown is the Type 20. This tube is an early form of the DeForest DV-6 and can be distinguished by the base marking of "PAT APL'D FOR" and the DeForest name etched into the top of the bulb.