The NU12 is the rectifier specification issued by the UK Navy to valve companies. The AP803 indicates Admiralty Pattern 803.
Commercially the specification is the equivalent of the FW4/500 full wave rectifier. This is a higher power device than most full wave rectifiers at 500 Volts at 250 mA. The valve is directly heated. The maximum reservoir capacitor is 16 µF and the minimum effective series resistance has to be 200 Ω.
The red paint looks like an indication that this was made by Mullard.
The twin rectifier sections.
One section face view with central heat sink flange.
The dome has the interference-fit top mica. The oxide coated ribbon filament is supported at the top by tensioning springs.
The round lower mica and pinch.
The classic envelope is 48 mm in diameter, and excluding the B4 base pins, is 126 mm tall.