It's a bottom bracket bearings preload collar/adjuster. Just like other bearing pairs (i.e. hubs, suspension pivots, headset), bottom bracket bearings need a way to remove slack, so spindle won't move sideways. There's a thread on the inner surface, so when you turn the collar, it will move left/right along the spindle. The bolt fixes the collar in place.
If you are looking for a replacement, I'd measure the axle diameter and collar thickness. If it's 30mm/5mm, I'd try a Race Face one, or contact any Shun vendor and ask about separately sold spare parts.
Some crankarm/BB systems don't use this exact method, and some do. For example, Race Face cinch crankarms have almost the same looking collar:
SRAM has a similar tool as a part of their BB30 and now DUB crankarms:
Sometimes there's not enough space for a collar, so other options to preload bearings are:
Optional stack of flat shims on spindle and a wavy washer, here's a screenshot of Zipp Vumaquad manual, figure 3 shows a wave spring:
Shimano's Hollowtech 2 crankarms use the non-driveside arm bolt to preload the bearings: