MOV were pioneers in HT rectifiers. U1, U2, and U3 were 'brights'. U4, U5, U6 and U8 were thoriated Tungsten 'dulls' and U5 had the additional distinction of being the first full-wave rectifier. U9 and early U10s exploited the azide process. while U12 and U14 were the first oxide-process types, although later production of U5, U8 and U10 also used oxide-coated technology. We do not know of a U7 or a U11, and we have been assured that MOV never made a U13; apart from this the MOV 'U' series ran continuously up to around U35 except where other brands (i.e. Mazda) got in first.
Initially the U12 and the U14 were the same valve (as were the MU12 and MU14). The design was intended to meet all HT requirements up to a maximum of 120 mA at 500V, using a capacitor-input filter. However, some valves performed better than others on final factory test so the best ones were graded and marked 'U14' whilst the also-rans were marked 'U12' and sold against a lower specification. This can be clearly seen in the photo of exhibit U12 where the normal etched factory type designation on the top of the envelope has the central type number omitted. The type number 'U12' is stamped (not etched) in large white letters on the side.
Exhibit U14 is of later (1940s) manufacture (note that the anode is made of sheet rather than gauze, indicating greater confidence in the quality of vacuum). By this time MOV had got their process sorted out and were manufacturing only U14s (which they supplied as replacements for U12s).