Hamish,
I had that same question when rebuilding my Jensen 541S from many boxes of bits. These cars came as standard with a fog and a spot light and the dash had two separate switches. I fitted a new AutoSparkes loom which had the two sperate sets of cables for these lights. When I positioned the loom the Spot Light wiring fell to the O/S and the Fog to the N/S so assumed that was correct!
Peter