This is an M-OV Type R from the early broadcasting era. The large style of BBC stamp carrying the words 'Type approved by PMG' was introduced in November 1922 but the phosphorous getter makes us think this example was made late in 1923.
The serial number is R9157. The base cap is nickel and the base pins are two part.
The horizontal electrodes and on the base cap is the letter G denoting the grid pin. The other side is an A for anode.
Looking through the anode. The side supports are solid whereas on the un-gettered R the grid support wires are twisted for strength.
The centre of the etched identification is Osram and to the right is the Marconi name - seen in the image above.
The microscope reveals the welded fixing points and the backbone support to the grid helix.
The balloon envelope is 55 mm in diameter, and excluding the B4 base pins, is 95 mm tall.
References: 1003. Type R Type was first introduced in 1916. See also1916 adverts.
