The 878A is an EHT rectifier designed for CRT power supplies. Contrast this rectifier with the Cossor SU2150A that was rated at 5 kV at 10 mA. Much smaller and used a lower heating power.
This exhibit has a small filament mounted high-up in the anode bell. Our other 878A has the filament closer to the base and appears to be longer in length. The filament looks to be thoriated tungsten by the rating and bright colour of the wire coil.
The green paint of the Maple leaf with the letters REL and R within.
The anode bell but no sign of the filament.
With the microscope and a part of the heating current. At full power the camera sensor is flooded.
The wide glass tube envelope is 51 mm in diameter and, excluding the UX4 base pins, is 158 mm tall.
References: Data-sheet, 1040 & 1043. Type 878A was first introduced in 1936.