the metropolitan police (met) introduced police boxes throughout london between 1928 and 1937, and the design that later became the most well-known was created by the met's own surveyor and architect, gilbert mackenzie trench, in 1929. initially, two competing prototype designs were installed on the newly built becontree estate in december 1928, with the winning builder being contracted to erect 43 boxes made of wood with concrete roofs in the final trench pattern as part of experimental installations in the richmond and wood green sub-divisions, which were completed in december 1929 and january 1930 respectively. their success resulted in the widespread adoption of the system throughout greater london over the next eight years using newer models of the mackenzie trench design now made completely of concrete for increased durability, save the doors, which were still made of teak. constables complained that the concrete boxes were extremely cold and damp compared to their wooden predecessors, so provisions were made for more powerful heaters. for use by officers, the interiors of the boxes normally contained a stool, a table with drawer, a brush and duster, a fire extinguisher, a first aid kit, and a small electric heater. like the 19th and early 20th century glaswegian boxes, the london police boxes had a light at the top of each box, which would flash as a signal to police officers indicating that they should contact the station.