The Lambda relay is marked with an X because in the older all-metal update kits all the relays looked the same, and this one is signficantly electrically different. If you put a common relay in that socket, the freqency valve will not run. If you put the "x" relay in most any of the other common-relay sockets, it will cause most of the grounds to melt in the relay compartment.

Hence the sign DO NOT INTERCHANGE MODULES.