The original A739 was, as its number would suggest, a close companion of the A738. It ran along Glasgow Street in Ardrossan, linking the A78 to the end of the A738, and onward to the harbour itself, just over a mile of length in total. Ferries still sail to this day to Brodick on Arran.
The A739 number didn't last long, and had been swallowed up as an extension of the A738. In 2004, the A738 took over the A78's route when the Three Towns Bypass (Ardrossan, Salcoats and Stevenson) was built and what was A739 is now B780. Given that the road to the harbour has always been classified, both the successors to the A739 have had a spur.