Chrome plated steel.  Bare cable guide on the right, for the rear derailleur; housing stop on the left for front derailleur. 

The securing screw and nut are rather interesting.  The screw is threaded full length, and has a combination phillips/slotted screwdriver head.  The 8mm hex nut is actually a sleeve or barrel with a hex head, also fully threaded throughout its length.  I suppose there is an advantage in that the barrel acts as a bearing, so the screw threads don't rub against the sides of the hole.  However, since the screw is threaded all the way up to the head, the screw threads on that side will rub.  Granted, it is more important to protect the threads on the nut end of the screw than on the head end.  Another advantage of this design is that longer threaded length distributes the force and reduces the chance of stripping.  However, I wouldn't think it necessary to tighten this clamp so hard that you'd strip the threads.  Note also, the hole on the right side of the clamp is oversized to allow the barrel to enter, which also means the screw can only go one way: screw head on the left, barrel nut on the right.  Finally, note that you'd better plan on using the Shimano fasteners with this clamp, rather than any old screw and nut.