This is a mystery. Clearly made by Mullard, it carries the identification characters of 1955 and MMTO R3C. The construction with two loops of wire has all the hall marks of a barretter. Given Mullard used 1909H as a barretter code it is quite possible that the Type is 1955.
The B12C base was used for compactron valves for, mainly, domestic television sets and as these had series heater chains the use of a current stabiliser on the heaters would be appropriate.
Side view showing the internal wire and supports.
Here the wire looks to be two separate loops.
Thanks to Andrew Rose for sending these images
The wide glass tube envelope is 28 mm in diameter and, excluding the B12C base pins, is 25 mm tall.
Updated January 27, 2021.