M-OV's first indirectly-heated HT rectifiers were the MU12 and MU14. These were in fact the same valve but only a minority of each production batch were good enough to withstand the full rating when tested. These few were labelled MU14.
The original production was enclosed in a balloon envelope
This exhibit carries the GEC label that was reserved for the professional market. When the shouldered envelope was designed with mica supports to hold the electrodes firmly in place the change was made but the pinch construction remained.
Face view of one of the anodes. The cathode is flattened and the heater is a single inverted V.
The classic envelope is 45 mm in diameter and excluding the B4 base pins, is 98 mm tall.
References: Data-sheet & 1040. Type MU14 was first introduced in 1933. See also1933 adverts.