The GP607 marks the point in 1928 when BTH, who had bought the Mazda lamp brand, started to use the Mazda brand for their valves. The GP607 is a general purpose valve with a six volt filament. A GP407 was also made for those with 4 Volt LT supplies.
The filament is oxide coated nickel.
This exhibit contains both the BTH mark and the Mazda brand. The BVA logo shows that the valve dates to after 1926 when the BVA came into being.
The base with exhaust tube in the centre. The centre lettering says BTH.
The balloon envelope is 44 mm in diameter, and excluding the B4 base pins is 110 mm tall.
Reference: 1043. Type GP607 was first introduced in 1928.