That, however, was the price paid for an early 19th-century German mechanical calculator.
'Quite simply,' said a Christie's spokeswoman, 'two people wanted it. It's as simple as that.'
It was bought by Edgar Mannheimer, a well-known Swiss dealer in clocks and scientific instruments, over the telephone. He engaged in unprecedented competitive bidding, battling against a German museum, whose representative was in the room: the atmosphere was electric.
As the final gavel fell, a spontaneous applause broke out, relieving the tension. Christie's described the final figure as the highest for any 'non fine or applied work of art'.
Admittedly, there has been nothing like it on the market for 70 years, and similar instruments exist only in museum collections - in Munich, Stuttgart and Mannheim. But Dermot Chichester, managing director of Christie's London, said: 'If we'd put an estimate of pounds 7.7m, people would have thought we were completely mad.'
He had rung the sleeping seller, a private collector, with the good news: 'He's on a different timescale from us, and I don't think he believed us'.
The gilt and lacquered brass piece (left), 8 1/2 in (21.5cm) in diameter, was made in the early 1820s by Johann Christoph Schuster.
A work of art as well as a working scientific instrument, its mechanism is so complex it took him two years.
It is a predecessor of the modern calculator: through a series of 20 enamelled dials, which the operator sets with a winding handle, it multiplies, subtracts, adds, calculates square roots and much more - apart from estimating auction prices.
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