+1
I'm on my phone and can't tell what year your DT is, but have you tried the schematics on powersportsplus.com? At least to get a part number maybe. The bracket looks just like mine except where the light assembly mounts.
Not that it helps, but my DT has the round taillight (437-84551-61-20; http://www.powersportsplus.com/parts/search/Yamaha/Motorcycle/1976/DT175C/TAILLIGHT/parts.html) and it goes on a few models (http://www.powersportsplus.com/parts/detail/yamaha/YP-437-84551-61-20.html)... So you can probably rule those models out. ??