The only flag with a chain on it
Gibraltar, and nothing else in the atlas. The sentence under it is where that is written down.
- Gibraltar
"An escutcheon on which the upper two thirds shall be a white field and on the said field set a red castle, and below the said castle, on the other third of the escutcheon, which must be a red field in which there must be a white line between the castle and the said red field, there shall be a golden key which hangs by a chain from the said castle, as are here figured".
Flag of Gibraltar on Wikipedia
How a flag gets onto this list
A flag is here because its own English Wikipedia article says so, in the sentence printed under it. Nothing was decided by looking at the picture: colours can be measured off a drawing and a lion cannot, so this list defers to a source anybody can check rather than to our opinion of what we are seeing.
That also means the list is short rather than wrong. Where an article never gets round to describing what is on the flag, that flag is missing from these lists even though the symbol is plainly there — and we would rather be caught missing one than caught inventing one.