The red 1 prong Orbit atom represents a terminal oxygen & is often used in conjuction with the 110 degree angled oxygen in carboxyl groups to denote resonance.
Indigo Instruments has held inventory of genuine Cochranes of Oxford atoms & bonds for over 30 years (scroll down to see "Skeletal (Orbit/Minit)". These parts are 100% compatible with every set & kit we've ever sold. Why buy from others where support could end the day after?
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.