In DDI-1, LK2-LK3 connect to the P1/P0 pins of the FDC9229BT Disc Data Separator (P0=LK3, P1=LK2, and fixed: P2=GND). According to FDC9229BT datasheet: ''P2-P0 select the amount of precompensation applied to the write data''.
== CPC6128:LK7 aka CPC664:LK201 - Autoboot CP/M ==
Same as DDI-1:LK1 (see there).