This is all the clutch brake consists of to stop the transmission flywheel.
The two red arrows point to the acutal brakes. When you push the clutch pedal in, a linkage not only pushes in on the clutch fingers to release the disc, but it also pulls a rod that then pulls the plate that puts pressure on the back of these pucks on to the back of the transmission flywheel to stop it. They are a little more forward than they will be in the tractor, I just pushed them forward a bit for clarification. If I had pushed them another three eights inch or so, they would fall out. There is no mechanical device that holds them in, they kinda 'float' there, sandwiched between the transmission flywheel and the plate.
Might be able to see it better here: http://i1195.photobucket.com/albums/aa4 ... 3fe102.jpg
Hope this helps.