These companies simply underwent numerous brand and operational name changes through the years under different ownerships. MWT (Marconi Wireless Telegraph), Osram (Edison General Electric), Genelec (General Electric), GEC (General Electric Company), Marconi Valve, Marconi Osram, English Electric, et al (I'm sure I missed a few) are all essentially the same company at different stages during history.
GEC-branded KT66 came in both clear and grey glass, black or brown base. The reason why most Genelec-branded KT66s are clear glass is because this was a marketing name used in later years (when only clear glass KT66s were produced). Similiar, the reason why ST "coke" bottled KT66s were mostly labelled as MWT or Osram is because these were developed and sold during the earlier years. Although brand names aren't an abosolute indicator of vintage, it can give some general insights as to when the tubes were sold.
GEC-branded KT66 came in both clear and grey glass, black or brown base. The reason why most Genelec-branded KT66s are clear glass is because this was a marketing name used in later years (when only clear glass KT66s were produced). Similiar, the reason why ST "coke" bottled KT66s were mostly labelled as MWT or Osram is because these were developed and sold during the earlier years. Although brand names aren't an abosolute indicator of vintage, it can give some general insights as to when the tubes were sold.