Are you sure the bike is as old as the title in your thread - looks a bit more recent to me? Lots of frames have bowed seat and chain stays - IF it was faulty it would appear "bent" rather than "bowed"
... from experience of cold-setting the stays will not be bowed - the "bend" if any will be at the brake bridge on the seat stays and the bridge behind the BB on the chain stays - that said, for the amount that would have been changed by cold-setting up to about 10mm any bend would not be visible to the naked eye.
Take a long piece of string and tie it off to one of the dropouts then take it up round the head tube and back to the other dropout.pull it tight and tie it off . then measure the distance from the seat tube to the string if its the same on both sides chances are the frame is fine