After multiple tries, the new LK cartridge inserted correctly and was recognized by the 7900. I filled the cartridge with PiezoFlush and did an initial fill, followed by a couple of nozzle check prints. Everything appears to be ok.
(On an aside note, I tried again to install one of the older LK cartridges after the new one was recognized by the 7900. After a couple of tries, the old cartridge inserted and latched into place correctly, but the 7900 did not recognize it for some reason. I tried resetting the chip on the old cartridge, but that did not fix the recognition problem.)