As I understand it:
K-BAR stands for "Knife, Browning Automatic Rifle."
As the BAR did not have a bayonet lug. BAR gunners needed a general purpose knife that others used their bayonet for. It is not actually a Marine Corp Specific item.
But Marines are far more attached to the design than the Army...
I carried one on my web gear in the 82nd Airborne div in the 80s.