A 'Point of Interest' article in the current UNDERGROUND NEWS (LURS house magazine) says:
"References in previous issues and elsewhere have perpetuated the misheld belief that the slang term for a Junction Route Indicator are 'Harbour lights'. However, the real term is ARBOR lights. To explain its origin, Arbor is the Latin term for 'branch'- your local tree surgeon is an arboriculturalist!"
I'd assumed there was a nautical connection as many 'harbour' entrances do use light signalling systems to permit entry and to indicate channels to be followed and that it was a 'Londonism' to 'arbour!