The K2s that remain (with cut-out crown regular windows - as pictured left) are listed 'buildings'. These were introduced in 1926 in Metropolitan Boroughs. The smaller K6s of 1936 (with raised crown irregular windows - as pictured right) were designed to commemorate the silver jubilee of George V and were sited in the whole country. K1s were non-standard, K3s concrete (by Scott) K4s included a stamp machine.