I've seen K used to designate normal mechanical relays in U.S. schematics too.
Not really relevant to modern computer boards, but one relay labeling system I do like is the old scheme used by the British GPO (and some others). Each relay is given an RL code followed by its own letter.
The coil on a schematic is labeled something like RLC/4. (It would actually be a horizontal bar with the RLC above and the 4 below it.) This designates relay C, and tells you that it has 4 sets of contacts. The contacts on the diagram are then labeled RLC1, RLC2, etc.
This was very useful in telephone work and any other similar relay systems where a coil could operate as many as 8 or 10 sets of contacts.
