Bet your problem was that counter guy didn't pull up the right engine family. Derringer has two different 6.7 Cummins listings in their system, one for the 2013-2018 and another for the 2019+ with the redesigned injector bore. If he just searched by engine size instead of the full CPL number, that's how you end up with cups that look right but won't seat. I watched the same thing happen to a buddy hauling a load back from the race track.