There is wide open space on JL51 HND-SYD but only if you search using the non-multi city tool on an itinerary that originates in Australia (that is to say, SYD-HND return)
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I'm unsure why it doesn't show if you use the multi city tool, even if searching with an itinerary that starts in Australia / has Australian POS.
This is real bookable space, I was able to call up last night and have them edit an existing booking (QF J award return) to be on JL51 on the return leg instead. This was just ticketed this morning:
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Unsure if the call centre can book it just as a one-way standalone.