Joined: Mar 2014
Posts: 18,000
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From: ,location, location
Bikes: Foundry Chilkoot Ti W/Ultegra Di2, Salsa Timberjack Ti, Cinelli Mash Work RandoCross Fun Time Machine, 1x9 XT Parts Hybrid, Co-Motion Cascadia, Specialized Langster, Phil Wood Apple VeloXS Frame (w/DA 7400), R+M Supercharger2 Rohloff, Habanero Ti 26
If they have to list authentic in the title that is not a good sign. Again Shimano doesn't sell through 3rd parties. Your local shop could get you another similar cheap derailleur with maybe fewer problems than tourney or not but in the end just save the money and put it towards a bike that makes sense for his riding.