Challen started to make the largest piano in the world for the Silver Jubilee of George and Mary, in 1935. There may have been two pianos or the one withdrawn repainted and polished and turned out again. There was a letter in the Piano Tuners Quarterly form Challen dated 19th December 1940, saying:
“Particulars of the world’s largest grand piano, manufactured by Charles H. Challen & Sons. This instrument was made in honour of the Silver Jubilee of their Majesties King George V and Queen Mary and is the largest grand piano in the world. It has created a profound impression in musical and engineering circles. It is eleven feet and eight inches long, weighs one and a quarter tons, and the combined tensile stress of the strings amounts to over thiry tons. The iron frame alone weighs six and a half hundred weight and the longest bass string is nine feet eleven inches.
The experimental work in connection with its production took over twelve months, and the cost amounted to over £600. In spite of its immense dimensions and strength, so perfectly has everything been calculated that it is even easier to play than an ordinary piano. It is a feat of British piano engineering, which places the British industry definitely on top. It was first shown at the British Industries fair in 1935, where it was inspected by H. M. the Queen, and was played for her by Billy Mayerl. At a later date Sir Walford Davies, Master of the King’s Music, also played the instrument and in reference to the piano he stated ‘it is indeed a glorious instrument. It has the bass of an organ and yet the intimacy of touch and tone of the finest piano I have ever played. A great achievement’. It has been used on several occasions for broadcasting.”
–Charles H. Challen