Already tried to identify it with no success: http://typophile.com/node/98764

This is one of a series of posters done by artist John Gilroy for Guinness in the 1930s and 40s. Like many color posters of that era, this one was probably hand-lettered. My eyes aren't as keen as they used to be, but I believe the two lowercase a's are slightly different. The artist was clearly using Gill Sans (1926) as a model. (This poster predates Candara by nearly half a century, and Humanist 521 is Bitstream's brand name for Gill Sans.)