Here's an excellent treatise on TOs, with some arcana on the 1L6 tube.
http://www.zenithtrans-oceanic.porta....co.uk/1l6.htm
But there's an error in the second to last paragraph:
...1U6 is a 25 ma tube instead of a 50 ma.This means that it will test identically to a 1L6 in a tube tester but when placed in a radio,
nothing will happen since it has double the filament resistance and the TransOceanic uses a series filament string.
The bolded line should be corrected to read:
"You'll blow the filament since doubling the resistance will double the voltage appearing across it." Probably not an issue though, since 1U6 is even rarer that the 1L6.