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 labeled MU14 but the 'also-rans' (initially the large majority) were labeled MU12 and sold against a reduced specification (350 V).
The Marconi paper label.
Face view of one anode.
The classic envelope is 45 mm in diameter and, excluding the B4 base pins, is 102 mm tall.
References: Data-sheet. Type MU14 was first introduced in 1933.