Cossor LF triode, "Specially designed for low frequency amplification".

This has 1440 printed on, presumably the serial number. It also has the ratings on the label:

Fil. V. 5.5 - 6V.
Fil. Amps :1A
Anode Volts 60.V.

(the :1A is how it is printed)

The valve measures 115mm overall by 45mm at the widest point of the envelope..