Today I've taken a closer look at the board. There are a lot of 3-pad parts. The first two pads are linked with a copper trace and correspond to the '0R' resistors and the middle and last pad are left unconnected and the markings are those of the 'NL' resistors. These seem to be 'optional' features that the user might want to change. Why 'NL'... possibly from 'no load', it's strange anyway