 Complete the following phrase:  The K series was an entirely new design by the Škoda Works company of Czechoslovakia. The original K1 model was in production by 1933, and was a successful export weapon, with sales to Turkey, Romania, and Yugoslavia. However, the Czechoslovak Army was not a buyer, but desired modifications to the weapon before any purchase. The newer weapon was titled the K4 and was accepted by the Czechoslovak Army as its standard heavy howitzer intended to replace the large variety of World War I era pieces still on inventory. In Czech service it received the designation 15 cm hrubá houfnice vzor 37 and had just begun production when Czechoslovakia capitulated to Germany in 1939. The Germans continued production for service in the Wehrmacht as the 15 cm schwere Feldhaubitze 37t or sFH 37t. The weapon was widely used, particularly on the 