This publicity slide from NASA in 1972 (?), explaining how wonderfully useful and flexible the Space Shuttle would be, has an intriguing image in the lower right.
(NASA MH72-5017, via David Portree)
The "international" use is illustrated with a globe showing the US flag, the UK flag, and then a white flag with a single vertical blue bar on the right.
Its an odd selection. As I understand it, the UK had not committed to any kind of detailed partnership in the Shuttle program, and when it did come in, it came through as part of ESA. ESA did not exist in 1972 - its precedessor ESRO did, and was involved in the early Spacelab plans, but I can't work out if it would have had its own flag or not. And again the UK was part of it, not involved independently.
Wikipedia indexes many blue-and-white flags, but none that match this layout.
So: is this one a placeholder? Is it ESRO? Is it someone else who was interested in being a Shuttle partner? And why the UK flag on its own...