Purloined from an MGB Forum thread
"Yes, those are beer kegs under the wings.
"In the lighter moments of WWII, the Spitfire was used in an unorthodox role: bringing beer kegs to the men in Normandy .
"During the war, the Heneger and Constable brewery donated free beer to the troops. After D-Day, supplying the invasion troops in Normandy with vital supplies was already a challenge. Obviously, there was no room in the logistics chain for such luxuries as beer or other types of refreshments. Some men, often called sourcers, were able to get wine or other niceties from the land or rather from the locals. RAF Spitfire pilots came up with an even better idea."