This set of 2 tethered objective lens caps from Vortex is designed for use with 42mm Diamondback binocular (2016 & HD versions) only.
They are independently tethered to each lens and offer additional protection for your lenses. These are available here as a replacement or spare for the ones originally included with the binoculars.
The inside dimension is approximately 1.891"
42mm refers to the specification of the binocular - eg 8x42, NOT the physical dimensions of the binocular.