text: The British order was cancelled on 4 October, leaving only US production, but RAF representatives in the US were able to persuade the Americans to build fifty bomb casings by the end of the year, to be filled in Britain.Four Lancaster bombers for trials were ordered then this was cut to one aircraft. During the Battle of the Bulge (16 December 1944 – 25 January 1945) any complacency about an imminent end to the war was confounded; the Grand Slam production order was revised again for 25 from British sources and 200 from the US. An expanded version of the Tallboy design was not feasible and the English Steel Corporation of Sheffield began work on "large boilers" in late 1944. Experiments with the composition of the bomb casing, a steel alloy of chromium–molybdenum, were undertaken to find one that could withstand the shock of impact. A concrete core, accurate to 1/10,000th of an inch was made, then a sand-covered outer mould was created, into which the molten metal was poured. 