The Marconi type R5V was a landmark valve in that it was the first British high-vacuum receiving valve to incorporate a sprung 'inverted V' filament of the type which later became universal. The inverted V sprung filament had been used in transmitting valves as had the flattened anode.
As far as we know all R5Vs were gettered to ensure a high vacuum and allow use with up to 120 V on the anode. This, and its (relatively) low Ra value (30,000 ohms) enabled the R5V to provide sufficient audio power to operate a horn loudspeaker at (just) acceptable volume (tens of milliwatts).
The R5V from Marconi-Osram was a general purpose valve, similar to the original R-type, and made from the early 1920s in considerable quantity. The differences from the R-type were a lower anode resistance and the 5.0 V filament but it is still a bright emitter.
Here the etched lettering can be seen clearly. The BBC stamp is the first type - dating this exhibit to between November 1922 and September 1924. The Type designation is presented as R5 with the V as a superior character.
The other lettering is Marconi Valve made at the Osram lamp works.
The flattened nickel cylinder anode, the inverted U shaped grid support and the filament with tension spring held on a support outside the anode.
A closer look at the main electrodes. The filament is still intact.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 96 mm tall.
References: 1005. Type R5V was first introduced in 1922.