The Western Electric 'Weco' valve came in three sub-types, green spot, as this exhibit, orange spot and red spot. In the UK the valves were made by STC and given the prefix 4 as in this example.
The filament is given as 0.8 to 1.1 Volts at 0.25 Amps. The anode voltage was quoted as 15 to 30 Volts for the orange spot, and 30 to 60 Volts for the other two types. The green spot was designed as a detector with the grid at -7 to -10 volts. All three types were introduced in 1925, had an rA of 25K Ohms, a mutual conductance of 0.22 mA/V and an amplification factor (μ) of 5.5.
The lettering is clear as is the green spot.
The thin glass tube envelope is 15 mm in diameter and, excluding the base pins, is 65 mm tall.
Reference: Data-sheet. Type 4215A was first introduced in 1923.