John Hupse's site confirms the rumours on the Philips UY42 having been uprated to take 250V mains. Since he mentions no source, it's still rumours and he made some kind of a mistake in the codes or years mentioned, but his site is generally well informed so I think there must be some truth to it.
According to
http://www.hupse.eu/radio/tubes/UY42.htm the UY42 was manufactured under 3 codes, which is indeed very likely. The 2 codes N7 and TM that I had already found, and a 3 digit code that Philips used up to roughly 1948-1950. On this site, the 3 digit code is erroneously mentioned as 71 with likely the last digit missing (the UY41 has old code 691 so the code for the UY42 starting with 71 is plausible).
It's also stated that all valves carrying one of the other 2 codes are good for 250V and that all Philips UY valves from 1954 on are good for 250V. This leaves a 5 year gap of uncertainty, but suggests that only the very first production of UY42 was for 110V only.
I still have to find the Service Bulletin I was thinking of.