The ZD2 is described by Triotron as: an output stage in two or three valve sets or first stage low frequency if followed by transformer coupled UD2. Triotron also describe the ZD2 in their 1931 booklet as a super power valve. A slightly improved version was issued in 1932.
The dome carries the identification as an etched image. This ZD2 was made in Austria.
The gettering covers most of the glass but from the base end one can see the horizontal bright anode.
A closer view of the electrodes. The filament looks to be a single V.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 87 mm tall.
References: Datasheet & 1043. Type ZD2 was first introduced in 1927.