This is the genuine Emlid Reach RS+ Survey adapter.
With a 5/8 inch diameter, 11-thread per inch hole in the bottom, and a 1/4 inch diameter, 20-thread per inch screw on top, it allows you to attach the Reach RS+ to a standard survey pole, and many other surveying attachments.
While one of these comes with every Reach RS+ receiver, it’s easy to misplace, or lose in the field. And without it, a whole day of locating could be lost. We recommend you order two extras, and keep them in your supply box, just for emergencies.