The LF407 is from BTH's 1928 four volt range. The design use was for AF amplification with the P415 in the output stage.
The interesting point about this valve is that it was produced after BTH bought the rights to the Mazda brand name and before the AEI merger that saw BTH concentrate on equipment and Ediswan take over the valve business and the Mazda brand name.
The dome has etched markings showing company, brand and BVA membership. No BBC logo.
Through the gettering can be seen the bright box anode and the grid. At the top of the anode is a glass insulator that holds two filament tension springs. The filament is four strands in an M configuration.
The balloon envelope is 43 mm in diameter, and excluding the B4 base pins is 101 mm tall.
Reference: 1043. Type LF407 was first introduced in 1928. See also1928 adverts.