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 remains of the Osram paper label. The anode is ridged and has holes in it.
The anode full face.
The classic envelope is 45 mm in diameter and excluding the B4 base pins, is 105 mm tall.
References: Datasheet & 1043. Type MU14 was first introduced in 1933.