Swallows and Amazons (Series of Novels)

She had fastened it on the little wire flagstaff where the blue flag had been

Frank Pouw, 13 November 2014

Arthur Ransome's description of how it was made:

Titty had taken the little flagstaff up to the farm, and had cut a triangular flag out of some of the canvas left over from the tents. Mother had drawn a swallow on a bit of paper, and Titty had cut one out of some blue serge that had once been part of a pair of knickerbockers. Then she had put the pattern on the white flag, and cut out a place to fit it. Then she had sewn the edge of the blue swallow all the way round into the place for it in the white flag. When she had done there was a fine white flag with a blue swallow flying across it, and it looked the same on both sides. Then she had fastened it on the little wire flagstaff where the blue flag had been, so that it was ready to hoist to the masthead.

Frank Pouw, 13 November 2014

At Wikipedia, the cover of the 1st edition shows what seems to be a white rectangular ~3:5 flag with a large medium blue swallow outline, flying to the bottom fly. However, according to Wikipedia, this cover is by Steve Spurrier, whose illustrations Ransome disliked so much, that the rest of the illustrations went unused. Eventually, Ransom illustrated all his books himself.
António Martins-Tuválkin, 27 May 2008

The third flag (aside from the Red Ensign) that is mentioned in Swallows and Amazons is one that Uncle Jim (Captain Flint) has brought back from Siam: a green flag showing a white elephant.

"Just look at his flag", said Roger, who, as usual, was at his post in the bows.
They had passed the point, and could see into Houseboat Bay. There lay the houseboat, moored to her big barrel buoy, and on her little flagstaff of a mast, accustomed to carry the red ensign, was a large and most unusual flag, blowing out finely in the wind. It was a green flag, and in the middle of it, nearly filling it, was a huge white elephant. The Houseboat Man, Captain Flint, had dug it out for the occasion.
"I know what it is", said John. "It's the Siamese flag."
"I've seen it before", said Mate Peggy. "He brought it back from the East last year."

Uncle Jim lives on a houseboat on the lake, and normally hoists the Red Ensign, but when the Swallows and Amazons declare war on him, he hoists his elephant flag. When the children attack, they haul down the elephant flag and hoist the Jolly Roger instead (a rectangular flag in Uncle Jim's flag locker). It would seem likely that the elephant flag would be of the same design as the red-and-white Siamese flag of 1855-1916 and 1941-45 (as Thailand in those years).
Miroslav Vaverka, 23 July 2005, Mike Oettle, 26 May 2008, and Eugene Ipavec, 30 July 2008

I've checked everything in the Thailand pages and under the keyword "elephant", and the only thing that bears any resemblance is the flag of Nakhon Nayok province. There are some white elephants on blue, but none on green:

• Historical Naval Rank Flags
• Flag on State Buildings

I'm inclined to call it either a mistake or a deliberate alteration by the author.
Eugene Ipavec, 24 July 2005

At Kleine Letters and Fantastic Fiction, the cover and title page of a later edition, «illustrated by the Author», show two crossed pennants (i.e. triangular flags): counter-bendwise a classic pirate pattern and bendwise a white flag with "black" (i.e. ink color) swallow silhouette, flying towards the hoist. This image seems to be some kind of logo for the series, judging for the many book covers in which it is at the top: Swallows and Amazons, Great Northern?, Missee Lee.

This same image, but colorized so that the swallow is medium blue, is at the masthead of Mario Becroft's Swallows and Amazons Forever page, where the same image is also made available in other sizes and formats.

The DVD case of a joint edition of two 1984 TV episodes of Swallows and Amazons Forever! — "The Big Six" (IMDB) and "Coot Club" (IMDB) — shows a modified version of the crossed pennants motive, with the swallow with a white belly and flying towards the fly (probably also in the series' merchandise).
António Martins-Tuválkin, 27 May 2008