Yeah, that certainly doesn't seem to be a BX-type cable clamp. The ones I've seen in a certain DIY warehouse are roundish barrels with a set-screw to clamp on the metal jacket and a lock nut for the box.

The opening at the end where the wires exit is designed to hold the red bushing in place.

And yes. They're packed in clearly marked bags that have BX Connector or some such thing printed on them. [Linked Image]