It's a bracket for a 'walkydog', a device that lets my dog run with me safely, a brilliant piece of kit. She loves the bike and has done 2,000 miles with me so far! I have been roundly told off by the LBS and my cycling friends for daring to put my old, battered, cheap pannier rack on my new shiny bike, so will remedy that as soon as I have recovered from paying for all this!
All in all, I can't recommend everyone highly enough for putting this together for me, it's been a joy from start to finish.