Brian is correct - the Kirk D200 bracket does not properly fit the D300.
The bottom of the D200 bracket on the front of the camera curves up slightly - and as a result the front of the D300 pinches aginst the curve of the bracket rather than being nicely seated. While it will attach to the camera, it does not fit properly.
Likewise the D300 bracket will not fit on the D200. I have tested the RRS version of the D300 bracket.
Here are sample images taken today. Sorry about the quality - I was out of cameras and was hand holding.
Image 1 You can see that the D200 bracket does not fit snugly. Yes - I could force it into place but it does not fit without excessive pressure.
Image 2 By looking at the side of the camera, you can see that the bracket is not aligned correctly. It is about 5-8 degrees off.
Image 3 You can see the two L brackets side by side. The one on the left is the RRS D300 bracket and the one on the right is the Kirk D200 bracket. As you can see, the front of the D300 bracket is different from the D200 bracket. You might be able to machine down the front of hte D200 bracket to make it fit, but the front edge remains problematic.