This AC044 was made in the early 1930s and is much more bulbous but had lower power and voltage ratings (200 Volts max.) than the later version AC044 in a shouldered envelope. Apart from the heater the characteristics are the same as the AC042.
An improved version was issued first in 1933. The valve was redesigned and uprated in the late 1930s in order to compete with the Marconi-Osram PX4 type. Early specimens of the redesigned Mullard type can be recognized by snazzy gold spray on the top of the bulb. Production was suspended for a period during WWII, PX4s (when available) being supplied for replacement purposes.
Designed for single end use, the high gain and reasonable power with low distortion would have destined this valve for quality applications, especially in the later versions.
The identification is on the dome and the absence of gold paint under the lettering suggests that this is for the early 1930s.
The glass is largely opaque but the anode can be seen in this image. The likeness to the early PX4 is clear.
The balloon envelope is 56 mm in diameter and, excluding the B4 base pins, is 116 mm tall.
References: 1931 datasheet & 1040. Type AC044 was first introduced in 1930.