An advertisement from The Aeroplane, 1932, by Bristol Aircraft. (Notice the logotype, discussed earlier here, bottom centre.) A neat design, the monochrome printing leads to an unanswered query – which nine countries?
Some of the flags are identifiable – several are confused as they are all arranged vertically, not horizontally – and only readable with a colour key (not provided) and two, behind the ends of the laurel wreath, are completely unreadable. For reference, we can read (L-R): unknown: Siam (now Thailand, or as identified here ‘land of the elephant’): unknown; one of two; Great Britain; the other one of two; unknown; Australia and unknown.
The operators of the Bulldog, from Wiki here, were: Australia; Denmark (fourth from left, going by the tone matching the blue in the Union Flags); Estonia (the white bar giving it, but inverted compared to the others!); Finland (one of the inner two, but the white of the Finish flag being represented reversed…); Latvia (presumably third from left); Siam; Sweden (the other near centre, probably right, if the blue is correct); and the United Kingdom. Could the countries have been better represented by their nation’s aircraft markings? Probably; certainly The Aeroplane’s readership would probably be more literate in the roundels than indistinguishable crosses and tricolours!
Odd Bulldogs were also obtained by Japan, the US Navy and the Spanish Republic (secondhand) and these aren’t represented here.
As a design, it works well, though the nine Bulldogs in a flight seems to be from a different artist’s hand to the rest of the design. And presumably the reader was meant to feel the width of the sales success, not know the countries! Enigmatic…
James Kightly, Vintage Aero Writer.
A Year of Aviation Illustration & Art 
The flag directly to the left of the Union flag , I would say is most definitely Sweden . The one to the right appears to have the colouring of the Danish flag, but the cross is a bit too heavy. The dimensions of the cross are nearer the Finnish flag, but that has a blue cross on a white ground, so it would be a negative image for that, which would be even more incorrect for that.
I found the more you tried to ‘read’ it, accounting for expectation of consistency, the less the set could be real. Good point regarding the ‘in flag’ proportions, but the shield shaping of the flags also throws off proportions as well.