Johannesburg - An Irma Stern painting, originally auctioned off to pay for Nelson Mandela's treason trial legal fees, has been sold for a record R17.5m.The painting was recently discovered in a modest London apartment where it was used as a notice board, with the owner apparently having no idea of its value.It is the most expensive South African painting sold this year, said auction house Bonhams.The Arab in Black sold at a Bonhams sale of South African art in London on Wednesday for £842 500 (R17.5m)."This powerful image from Stern's highly regarded Zanzibar period, is one of the artist's finest works. It also has a fascinating past - from an important role in the political history of South Africa to its recent fate as a notice board in a modest London apartment," said Bonhams head of South African art Hannah O'Leary.Arab in Black was put up for auction in the early 1960s to raise money for Mandela and his co-accused's treason trial's defence fund to raise money for legal fees and support their families. Stern donated a work to this cause.The painting's original owner was art collector Betty Suzman, whose father, Max Sonnenberg, MP, founded the chain store Woolworths.Betty Suzman was anti-apartheid activist Helen Suzman's sister-in-law."In the 1970s, it came to Britain when the buyer emigrated to the UK and was subsequently bequeathed to the current owner. For many years, Arab in Black hung in a London flat and was used as a notice board."It was discovered during a routine valuation."I was undertaking a routine valuation when I spotted this masterpiece hanging in the kitchen covered in letters, postcards and bills," said O'Leary.Now Arab in Black is among the top ten South African paintings ever sold at auction.In 2011, Bonhams sold Stern's paining Arab Priest for more than £3m, setting a new world record for Stern.