The schematic just shows the one square one with the rounded corners under the spool tension knob... so, no, probably this isn't where it goes, I guess.
Would it fit inside the nut/cap that holds the line guide pin (pawl) in? If so, there should be a 'washer' of some sort in there to allow the line guide pin to slightly turn as it follows the grooves in the level wind gear. The pawl shouldn't turn on the cap itself; might cause some problems if it is like that, I would think.
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Putter