The TDD4 is an indirectly heated double diode triode for mains superhet receivers. It would have been used for signal and AGC detection and then as the first audio amplifier.
The reverse is plain metal loaded paint.
Looking into the top reveals that the triode has heat sinking fins on the top of the grid supports. Looking closer shows that the anode is cylindrical. The diode anodes must be at the lower end closest to the pinch.
The classic envelope is 36 mm in diameter and, excluding the B7 base pins, is 99 mm tall.
References: Data-sheet, 1040 & 1043. Type TDD4 was first introduced in 1933.