CNC machined 1-piece dual rear cog (18T and 16T) allows a gearing choice without removing the rear wheel. Just loosen the axle nuts, slide the wheel, move the chain to the other cog, set the chain tension, and re-tighten the rear axle nuts.
It's lighter than 2 separate cogs, and don't worry about alignment. It can only be used with a 3/32" chain, which has plenty of flexibility to handle a few mm of lateral positioning.
Another nice feature is that it allows yet another option for the other side of the hub, assuming it's a flip-flop hub.
Supplied with its own lock ring.