A tachograph is a device that combines the functions of a clock and a speedometer. Fitted to a motor vehicle, a tachograph records the vehicle's speed and the length of time that it is moving or stationary. The mechanical tachograph writes on a round piece of paper which constantly turns throughout the work day. The marker moves nearer to or further from the center according to the driving speed. An entire rotation encompasses 24 hours.
Analogue tachographs record the driver’s periods of duty on a waxed paper disc. However, these are vulnerable to tampering, and so are being replaced by digital tachographs which record data on smart cards.
Use in hitchhiking
Before most carriers banned drivers from picking up hitchhikers in the UK many truck drivers would give hitchers the waxed paper recording disc or tacho. The hitchhiker would display the tacho when trying to get a lift as a sign that they were approved of by a previous truck driver.
Although it has become difficult to catch a lift from HGVs in the UK these days it is still possible and your chances improve with the older drivers if you have one of these discs. You may get a lift out of pure nostalgia or curiosity.
 Times article on hitchhiking that mentions Tachograph discs.