My own feeling is that they are probably stonecutter's marks or road laying crew signs. Maybe two symbols show either the end of one cutter's work and the start of another's, or are the foreman's marks from a particular gang of workers either shaping or laying the stones. I wonder who they were and where the stones were quarried? A visit to the remains of the stone quarries on Dartmoor will show half-finished kerbstones lying around in the wild landscape. It must have been a tough life being a stonemason up there, are those weather-beaten quarries where these stones were originally shaped?
On New Oxford Street, these signs were all carved within a run of fifteen kerbstones, with whole streets nearby not showing a single one: