Originally Posted by bassmeknik
Probably a lock washer meant to be underneath the jack plate (between the jack and plate) to try and prevent loosening.
And if the lock washer has "teeth" it is meant to bite into the metal of the jack plate to make a better connection. It's always a bad sign when you have parts left over when repairing something!
But leaving it off isn't a biggie (jack may come loose again sooner than with it), but you do NOT want to leave it inside the bass!
Voice of experience says, it will rattle around in there and at the WORST possible moment it will wedge itself into the wiring shorting out the bass and causing it to quit playing.