so i bought a pretty messed up 65ish harmony bobcat that im restoring and since its hard to find OG harmony bobcat hardware im going for mostly all jaguar/jazzmaster stuff. when looking at pictures of the tremolo caveties there is a little round thing on the top of an otherwise rectangular shape, what is the purpos of this? on the jaguar tremolo i have there is nothing i can see that would call for the extra little knob on top. any help would be appreciated, im new here but look forward to contributing to this forum.
picture of cavity: 
