What is the tiny metal disc inside the string lock for? Do I need it?
The tiny metal discs that we fit inside the string locks at the headstock are there to protect the strings from the star grove on the mounting screws underneath the strings. Are they necessary? The answer is both yes and no depending on what specific string is in question. The lighter gauge plain strings are the ones most susceptible to damage and thus, there's where the discs are mostly needed. From the beginning, we only fitted discs in the locks for the plain strings, but as this turned out to be a source of confusion, we started including discs in all the locks.
The discs are not an absolute necessity and they do reduce the amount of available space inside the string locks. If you have problems fitting your desired string gauge on your guitar, please remove the disc for the string in question to access additional mounting space. To remove the disc from the hole, just flip the guitar over and it will fall out.