The NU7 is an interesting rectifier as the mercury can be clearly seen on the inside of the envelope. The heater is a thick coil of ribbon and the anode is a curved plate.
The civilian prototype of the NU7 is the M-OV Type GU1, introduced in the mid 1930s. The GU1 was very popular as a rectifier for cinema amplifiers (etc.) but it had a short life and was discontinued in favour of the GU50.