Santa Cruz rear derailleur hanger. This came with my Santa Cruz bronson CC as a spare but I sold the bike 6 months ago and only just found the service bag. If I remember correctly it was boost so this will be for 148mm rear axle fitment. I have measured the width of the hanger and it’s just under 20mm wide so you can check yours and make sure it’s the correct one before buying. Looks excellent quality made however I’m not sure if it’s oem Santa Cruz.
eBay