The A577 is an indirectly heated triode designed for use in valve voltmeters. It is designed to be used as an anode bend rectifier in the measurement of RF and AF voltages.
The Osram label.
The mesh anode has flattened sides, the grid wires pass flat across the face of the flattened oval cathode tube. The insulated heater passes through the cathode four times.
Use is made of glass beads to insulate the grid support from the cathode top support.
The balloon envelope is 50 mm in diameter, and excluding the B4 base pins is 107 mm tall.
References: Data-sheet, 1043. Type A577 was first introduced in 1936. See also1936 adverts.