Thanks for setting out that nice explanation of the dual inventory checks.

These exist also on international fares and we had to spend a few hours reverse engineering this to make a YVR-SEA-LHR D1 itinerary work. Someone in the premium fare thread on that particular good deal mocked me when i presented my results and referenced the eco fare bucket letters in the Z fares we were trying to book. “These are not Yup fares” the poster said, but a dual inventory check is just the other side of the coin of course.

Wish we had understood this before but it was fun to reverse engineer with a spreadsheet of Delta.com results and EF fare bucket searches. Once we figured this out we easily found flights for 5x pax, and are looking forward to a visit to London next week and our first D1 flights in 2.5 years.