The HF red ring was introduced by Mullard in 1924 as an RF amplifier or detector. The red ring around the top of the envelope. Mullard also made an LF for audio use and externally the difference is a green ring around the top of the LF.
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 etched markings on the envelope include the third and final BBC stamp. This places this exhibit as being made between 1924 and 1927.
Although difficult to see the shape of the anode and grid can be seen above the Bakelite base.
The balloon envelope is 35 mm in diameter, and excluding the B4 base pins is 88 mm tall.
Reference: 1003. Type HF was first introduced in 1924.