The Cossor W1 is a dull emitter triode and was introduced in 1924 as a replacement for the bright emitter P1. It was intended as a detector and/or audio amplifier.
The base is distinctive with an open area between the pins.
The W1 arrived it its original box and because the glass is silvered the only indication that the valve used the fan shaped anode is on the box. In fact the W1 was made just like the P1 but with an oxide-coated filament.
Like most early valves the operating conditions are printed on the box. Many valves of this period were sold to experimenters and home constructors. Additionally most commercial wireless set construction was carried out by small businesses.
The wide glass tube envelope is 28 mm in diameter, and excluding the B4 base pins is 75 mm tall.
References: 1003 & 1004. Type W1 was first introduced in 1924.