This Cossor rectifier is a full wave device with a directly heated oxide coated ribbon filament.
Testing the filament shows that it is a 4.0 V 1.0 A type and that strongly suggests that this is a 412BU.
The oxide coated filament ribbons are held from the top mica in tensioning springs.
The etched lettering on the dome carries the Cossor name and makers codes. The Type designation was contained within the paper label the remains of which can be seen on the right.
A close-up of the top mica and ribbon filaments.
The balloon envelope is 54 mm in diameter, and excluding the B4 base pins is 112 mm tall.
References: Data-sheet, 1040 & 1043. Type 412BU was first introduced in 1929. See also1929 adverts.
