I grew up in the 50's and 60's and water slide decals were everywhere like the vinyl stickers today. TG&Y was the neighborhood dime store and they had a little bit of everything. They had a pretty good section on the wall with a peg board full of packaged decals, from small ones like those flames to large ones used on furniture or other big projects. Websters Hobby House was 10 or so doors down the street and next door to Pat's Schwinn shop. Websters had lots of packaged water slide decals including generic flames. In the mid 70's I built my Daughter a rocking cradle and my Son a huge toy box and they were painted and then dressed up with cool water slides from TG&Y. Even today you can get some generic flame water slides at Michaels art and hobby store.
I bet every local five and dime across the country had those generic flame decals back in the day, so it doesn't surprise me to see the identical ones planted on these old bikes.