Item in question:

Sprung, Snap-ring type of object with 'D' section profile and a split to enable fitting to a shaft with suitable groove. When fitted to shaft, it enables the shaft to easily snap into (and out of) 'female' groove in hole. The 'D' section is to allow a gentle lead in/ out.

Basically it's a ring with a split in it.
Is this a standard type of retainig device (like a snap ring), available in standard sizes?