The Pit-Prop Syndicate. by Freeman Wills Crofts

The Pit-Prop Syndicate is a story from the beginning of the golden age of crime fiction. Seymour Merriman, a British wine merchant on business in France, happens upon a syndicate manufacturing pit-props-beams used to prop up mine tunnels-but his eye is caught by one odd detail : their lorry's numberplate mysteriously changes. With the help of his friend Hilliard from the Excise department they dig deeper and uncover a dangerous conspiracy.
Freeman Wills Crofts was a civil engineer, turned author of crime fiction. Though somewhat forgotten today, his style was widely appreciated at the time, and still finds fans of those who like a puzzle where all the loose ends are tied up. During his career he wrote over thirty crime novels ; The Pit-Prop Syndicate, published in 1922, was his third.