Two detection vans with loop aerials on the roof and a General Post Office (GPO) 'Wireles Investigation Service' logo on the side. The poster says 'Warning: Is Your Wireles Licensed?' Men are adjusting the roof aerials. Some of these vans were used by Post Office engineers to test subscribers' mains lines for interference. Most of the vans had standard bodies but from 1948, the Morris vans used for this purpose were fitted out as mobile offices with equipment to detect interference and unauthorised transmisions. The lettering on the cab door was applied to a removable metal plate to allow these vans to operate anonymously. The government first introduced radio licence fees in 1904, and these were not abolished until 1971.
© Daily Herald Archive / National Science & Media Museum / Science & Society Picture Library