This is an M-OV Type R from the early broadcasting era and carries the Marconi & Osram brands in the logo. The base has an outer bright shell. The large style of BBC stamp carrying the words 'Type approved by Post Master General' was introduced in November 1922.
This exhibit came in an NR13 box. The NR13 would have been the same valve but on an interservice base.
Sadly the pip has been broken and the valve is open to the atmosphere and some corrosion is developing.
Two stout supports are moulded into the pinch. To one is welded the anode cylinder. To the other is a pair of supports that form a rectangular support to the grid. This construction gives a more rigid grid and without the fear of sagging it can be made of smaller diameter and closer to the filament. The mutual conductance is thereby improved.
The anode/support weld in the centre. The filament is crimped at the right end and this provides some strain relief between hot and cold lengths.
All of the images of this exhibit have been optimised to bring out the etching on the envelope.The BBC stamp is in the foreground with the R to the left.
The balloon envelope is 54 mm in diameter, and excluding the B4 base pins, is 90 mm tall.
References: 1005, 1003 & 1004. Type R Type was first introduced in 1916.