The black 2 prong linear, 180 degree, Orbit atom can be used to represent carbon in triple bonded structures; the triple bonding can be implied or if used with an "X" peg can show the 3 bonds.
You can use these Genuine Cochranes of Oxford Orbit Molecular Model Parts to replace missing pieces or augment any of the Orbit Molecular Model Sets & Molecular Model Kits supplied by Indigo Instruments.