They are designed to fit some of the rotors available from Hi-Spec motorsport http://www.hispecbrake.co.uk/page485.html (300mm I think) and have a floating mount which allows the alloy rotor and steel disc to expand independently when hot thus preventing cracking or warping. I have a set of mounting bobbins which were specially made for me at a cost of £10 each (there are 20 of them!). I only have bobbins for one pair of bells but I could provide the details of the guy who made them as he still has the tool he made to produce them.
will also fit Sprint/Sud with outboard front brakes (might fit the inboard ones but not certain)
sensible offers...