Apologies for the confusion there. When I was searching for inventory on EF, I wasn't searching for specific flight numbers and instead, used the Award and Upgrades search to search for all flights between EBB-LAX using KL as the airline and selected the "Business Upgrade with Delta SWU/Global Upgrades" option. The search resulted in two options, one on KL metal (KL 537 to AMS and KL 601 to LAX showing Z1 on both legs) and a second option with mixed metal (KL 537 and KL 6068 operated by DL showing Z9 on both legs). When I originally searched using the same parameters prior to booking the flight, the first option showed Z3 on both legs of the outbound and the second option showed Z9.