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.
Initial production was in this balloon shape but later production moved to the classic style as in MU14.
The anodes are made from wire woven into a mesh and supported on stout wire supports. The top of the anodes are held in a mica support. Note that the mica is not held by the envelope as would be the case with later envelope designs.
The osram label on the envelope and within is one of the anodes seen face on
The balloon envelope is 56 mm in diameter and excluding the B4 base pins, is 118 mm tall.
References: Datasheet & 1043. Type MU14 was first introduced in 1933.