[Maarten]

In the "Philips Service Maandblad" October 1949, Philips warns against using a UY42 at 220V in BX180U and BX182U sets. They also state that using a UY41 at 127V isn't a problem but will cause slightly less performance.

1949 is sufficiently close to the end of the old code usage that most or all UY42 on the market would have been of the 71x variety, so this is in line with what's mentioned on hupse.eu, even though I haven't found any mention yet in later documentation, of the UY42 from then on being suitable for 220V mains, but I've found numerous later sets that both have a UY42 and a 220V setting.

Though the exact year is as of yet unknown and the codes unconfirmed, it seems plausible to me that Philips at some point in time, made the UY42 suitable for use on 220V mains.

Also, the successor valve UY85 requires a series resistor when used on 220V mains, but it may be omitted at 127V. Maybe it isn't (only) the voltage but also the low internal resistance and resulting higher surge current that made the UY42 unsuitable for use on 220V mains.