Molded joiner sleeve. Use to attach 10mm square boom stock end to end. Can be used to lengthen booms or provide for dismantling. Perhaps you have a camera boom you want to be able to remove easily? Use a stub boom in the Quad, cross drill it and this part for easy reconfiguration. Fasteners not included, part is un-drilled. Discontinued by Millennium R/C, sorry! See Millennium R/C X-Fuse Parts Spru for this part linked here: https://www.radicalrc.com/item/Millennium-RC-X-Fuse-Plastic-Parts-Spru-107009 SKU Number: RRCSSBMJNR
 "Boom Joiner 3 Pack")