This is an Osram Type R. The envelope has the etched letters RAF dating it to at least April 1918 but no BBC stamp and thus probably before 1922. It also carries a number 26151 and the RAF up arrow. The other side of the envelope carries the Osram name, GEC and Made in England.
On the left the single strand tungsten filament has a coil at the start to allow for expansion. The coil is also seen as a shadow on the lower left of the image. The twisted wires on the right form the grid supports. A thin wire runs between the supports to hold the grid winding. To hold the grid in place a fine wire wraps around the turns and the horizontal support wire.
The filament is intact and passes close to the centre of the grid helix.
In this image the filament coil is clearly seen with the tungsten wire held in the fold of the support. The end of the grid support is also folded to hold the grid wire and the support wires.
The anode is formed as an open cylinder. The outer part of the cylinder is welded to the support strut that looks to be formed as an open triangle or angle.
The balloon envelope is 53 mm in diameter, and excluding the B4 base pins, is 88 mm tall.
References: 1005, 1003 & 1004. Type R Type was first introduced in 1916. See also1916 adverts.
