[Arran]

I think it depends on the particular front end of the radio but in most cases a 1R5 will work as a substitute, though some suggest that two of the pins need to be rewired, or a pin cut off in certain cases. Brand Z had a number of single band sets that also used used the infamous 1L6, my guess is that they did so to achieve some economy of scale since they were the ones who contracted with Sylvania to manufacture the 1L6 to start with.
The story I read is that Brand Z spent a lot of time, and money to develop the pushbutton activated front ends for the T.O sets in the mid 1940s, and they designed it to operate with a 1LA6, by 1949 seven pin miniatures (finger lamps to our former Soviet friends) were the tubes everyone was using. So rather then redesigning the T.O front end from scratch, Brand Z approached Sylvania and had them make a seven pin mini tube with similar interelectrode characteristics as the 1LA6, and so the 1L6 was born.
I have a G-500 T.O and the chassis looks like it was originally punched out to accept octal/loctal based sockets as all of the socket holes are 1-1/8'' rather then 1/2-5/8 of an inch. I honestly can't see why they bothered, even on the 600 series, they didn't make the chassis, or the set any smaller, it's still a chunky suitcase sized thing, and was until the quit making them in 1961.
Regards
Arran