History. No count from the ten: their second dates came one to seven days after the first, and this one comes 21 days after.
Hunch. On 18 May the RMT dropped dates it had named weeks ahead, and 17 December leaves both sides 41 days to talk.
The 17 December market resolves YES if before noon London time on 17 December 2026 the RMT has not withdrawn, suspended or moved this date and its drivers start the strike that day, whatever happens on 26 November, and NO if the date is withdrawn, suspended or moved before noon or a court stops it. TfL's strike page that afternoon with the RMT's statement settles it, confirmed by BBC or The Guardian, which decide alone if the page is silent.
We looked for cases where a British union named strike dates in a dispute in which it had already dropped one strike and gone ahead with another. Ten fit, all of them RMT strikes. Five were on the Tube and five on the railways. Each named two or more dates at least ten days ahead, with no other union for the same staff shown striking with it at that point.