[Arran]

Russ;
 I wonder if the more ubiquitous 1R5 also suffers from this, they used to be notorious for refusing to oscillate if the filament voltage dropped 1/10th of a volt? I have an RCA portable, one of those late 40s ones with the aluminum case, I think it has four bands, RCA used a 1A7 for a mixer-oscilator, even though the single band models used a 1R5. I'm starting to get the impression that the 1L6 was what one British professor of engineering (architectural engineering not electronics) would call "an innovation without development".
 From what I read the only reason it came into being was because Brand Z wanted to modernize their T.O chassis to use 7 pin miniature  tubes, and contracted Sylvania to enter a crash program of stuffing a 1LA6 into a 7 pin mini envelope, so they wouldn't have to re-engineer the front end. Why they bothered with this makes no sense from an engineering standpoint, they never downsized the T.O series, nor the chassis to match, so it had to be for marketing purposes alone. I have one of the G500 sets and the chassis holes were punched to accept normal size tubes, so clearly it was a last minute alteration.
Regards
Arran