Is this a crown race?
Yes, the piece in the lower left is the crown race. Split types (with the slit) should gently pinch onto the crown race seat of the fork. What's important with that type is that there's zero play (slop or free movement) between the race and the fork. It will be able to turn but shouldn't be able to just spin freely.

If there is play or free spinning, you could go a couple different directions. You could measure carefully (with a caliper) and determine whether the race internal diameter is oversized and/or if the fork's crown race seat is undersized. Then you could get a different one of whichever is causing the problem. 

Alternatively, if you just want to make the parts you have work, a good approach here may be finding a shop that has a Stein knurling tool, which is a very fast way of slightly raising the surface of the material, for round parts that are supposed to fit just a little bit snugger than they are.

The other piece is the compression ring, which goes between the upper bearing and the top plate.