It was estimated at £1000-1500.
Downing Street was the 'ace' address among a cache of hundreds of signs that were being sold on behalf of Westminster City Council, who are upgrading all signposting as part of the Legible London campaign designed to help pedestrians walk around the capital.
That includes many of the familiar 31 x 17in (43 x 80cm) City of Westminster signs designed in 1967 by the Russian-born architect Sir Misha Black (1910-77) plus some of the more recent cast iron and aluminium black and gilt 'finger post' signs - each sold with a certificate verifying its origin (despite copyright law they have been much reproduced).
The Downing Street sign was bought by David Soskin, who was former special advisor to John Major in the No. 10 Policy Unit - who lives locally to the saleroom.
At the sale on October 22, he had to compete against several UK and international telephone bidders.
Remarkably when the first tranche of signs was sold by Summers Place in May another Downing Street sign took a 'mere' £7800. Proof of the unpredictable nature of the auction process was that this one took over four times that sum.
The buyer's premium was 25%.