British Type "A" Tube

The British type "A" tube was introduced in 1917 by the British Thompson-Houston (B.T.H) Company.  This tube may have been designed for use in low power transmitters.  The notch in the press visible in the second photograph was typical of tubes designed for operation at high voltages, and the double-cylinder plate may have been an attempt to increase the surface area for better heat dissipation.

Unfortunately, the tube shown here has been damaged.  The internal elements are out of position, and the tube has lost its vacuum, which explains some of the discoloration inside the bulb.