Do I are have stupid? (Comet RS-720NMO/CK-3NMO3)

Hi all,

Yes, I do are have stupid, but on this one specifically. I bought a CK-3NMO3 coax set to put in my RS-840, thinking it was the NMO variant. It was not. So I bought an RS-720NMO from GigaParts to put the wiring in, and when it got here, it didn't fit. What gives? Is there a different coax set that fits in the 720NMO? I'm supremely frustrated by the whole thing.

Thanks,
Mairin, AB9UZ