That looks like more drop than 50mm, you may need a long drop. Also not clear from the pix whether the fork crown is drilled for an allen key type or whether you need a nut fitting. If it's both long drop and nut fitting, the brake you need is available but not commonly held in stock by shops or even wholesalers, due to the limited call for it. SJSC have a brake which will fit, whatever kind you need http://www.sjscycles.co.uk/category.asp?src=lfgss&cat_id=601
That looks like more drop than 50mm, you may need a long drop. Also not clear from the pix whether the fork crown is drilled for an allen key type or whether you need a nut fitting. If it's both long drop and nut fitting, the brake you need is available but not commonly held in stock by shops or even wholesalers, due to the limited call for it. SJSC have a brake which will fit, whatever kind you need
http://www.sjscycles.co.uk/category.asp?src=lfgss&cat_id=601