Here's a couple of enlargements of Illustration 4 from the Tractor 11 Parts List, which Grumpy posted earlier. Also the Part numbers for that illustration.

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Of course, there may have been several variations on different models.

#24 is the "Cutter Brake" and #23 is the Brake Lining material attached to it.
#30 is a Spring, which presses the Cutter Brake against the edge of the Pulley #36. This is the Cutter Pulley which sits on top of the Spindle Housing #39 (I'm assuming you realised that it is this pulley the Cutter Brake pushes against).

The Cutter Brake pivots via the mounting hole you can see on its left side in the pic.

#27 is a Belt Guide Tag, just a strip of springy steel, which probably sits against that Cutter pulley, directly opposite where the Cutter Brake goes.