The six [[GX4000 cartridge|cartridge]] LKs are for A15,A17,A18 (address lines, of course, not the "A1-A18" pins).
VCC ---LK1--- EPROM.A18---LK2---CPC.A18CA18 VCC ---LK3--- EPROM.A17---LK4---CPC.A17CA17 VCC ---LK5--- EPROM.A15---LK6---CPC.A15CA15
Some cartridge PCBs don't have any LKs installed, instead, the etched circuit has hardwired connections between some of them. One could scratch them off, and then use the LK soldering points to reconfigure the board for eproms of other size.