In the mid 50s, the class 50 engines had boilers of St 47 K steel. However, a total of 208 engines were recalled and fitted with new boilers and refurbished in accordance with the DR general reconstruction priciples between 1958-1963. These reconstruction measures changed the appearance of these locomotives (designated as class 50 Reko) measures significantly, especially with respect to the boiler and boiler periphery.