Princess Diana’s iconic red sweater with a black sheep on it is going up for sale in August, luxury auction house Sotheby’s announced Monday, after being discovered in storage earlier this year. Diana famously wore the sweater at Prince Charles’ 1981 polo match, bringing national attention to designers Sally Muir and Joanna Osborne and their knitwear brand Warm & Wonderful. Osborne told PEOPLE that she stumbled across the old sweater in her attic after forgetting that Diana had sent the damaged garment to be replaced years prior. “It was well preserved, and I had a kind of Groundhog Day moment and thought, ‘Could this be the one?’” she said. Sotheby’s called the sweater “meticulously preserved.” It will be offered during New York Fashion Week for an estimated $50,000 to $80,000.