I suggest that it is the iconisation of the Stubbs portrait of George (later the) IV’s most successful horse, ridden in a distinctive flying gallop by his favourite jockey, Old Sam Chifney; and that the depiction of boots and reins without a rider symbolises George’s disgust at the termination of Chifney’s career by the Jockey Club following a race-rigging scandal.
How a mass-produced 1960s joke sign for bars and pubs became evidence of historical authenticity before being censured as a hate crime against tinkers and thinkers (or organ-grinders, as we are often known).