John Gilroy was the illustrator of all those wonderful Guinness ads during the 1930s-1950s. Gilroy created an oil-on-canvas painting as a final proof for each ad. After the ad was approved or rejected or whatever, the canvas would be rolled up and stored away. This went on from the 1930s until 1962, when Gilroy stopped working for Guinness.
Sometime in the 1970s, a secretive collector bought the entire cache of Gilroy canvases. A few years ago, the Gilroy canvases started making their way onto the art market. Now, former Guinness brewer and Guinness authority David Hughes has written a book about the canvases called 'Gilroy Was Good For Guinness.'
(thanks Ben)