The TDD2A is a double diode triode for battery receivers. The function was as detector, AGC rectifier and first audio voltage amplifier in a superhet broadcast receiver.
We also have the earlier balloon bulb TDD2A version.
This exhibit has the modern top cap that was introduced in the early/mid 1930's. The filament is a single inverted V held at the top in a tension spring. The anode is a rectangular box and the grid wire passes flat across the filament. With the triode at the top the diodes must be near the pinch.