The HF was introduced by Mullard in 1924 as an RF amplifier or detector. Our other examples have a red ring around the top. The similar LF has a green ring around the top. One suspects that the difference was only found on test and the appropriate ring painted on.
This exhibit lacks any rings and the third type of BBC stamp dates it to before the end of 1927.
Internally the construction is unusual. The anode is an inverted U section mounted horizontally. Within the anode the grid is also U shaped. The filament is a fan shaped loop. In many respects this construction echoes that of the Cossor P1 and P2.
The feint Mullard logo etched into the glass.
The BBC stamp and letters HF are on this side.
The enhanced image shows the third type of BBC stamp.
An improved version of the Mullard logo and a partial view of the horizontal electrodes.
The balloon envelope is 33 mm in diameter, and excluding the B4 base pins is 84 mm tall.
Reference: Data-sheet & 1003. Type HF was first introduced in 1924.