Well, here's the "Home Service" album on which it appears Wild Life, so it probably does exist. Presumably it was a reference to navvies as (from here)

[Their] distinctive clothing included "...moleskin trousers, double-canvas shirts, velveteen square-tailed coats, hobnail boots, gaudy handkerchiefs, and white felt hats with the brims turned up. They would pay fifteen shillings, a great price, for a sealskin cap, and their distinct badge was the rainbow waistcoat."

Can't tell youy any more though ...

Cheers!
Ian