The AC044 in the photo is a genuine Mullard-made specimen, dating from the late 1940s or early 1950s. AC044s made in the early 1930s were much more bulbous but had lower power and voltage ratings (200 Volts max.), lower μ and lower mutual conductance. 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.
The lower end showing eight oxide coated filament strands.
The top mica and filament tension springs.
The classic envelope is 51 mm in diameter and, excluding the B4 base pins, is 131 mm tall.
References: 1931 data-sheet & 1040. Type AC044 was first introduced in 1930.