Sparky Watts started as a newspaper strip in April 1940 from the Markey syndicate. It ran for two years until Rogers went off to fight WWII. The original appearances in Big Shot were reprints form the strip I believe beginning in BS #14. The first four (?) issues of SW were also reprints. It's possible that Markey licensed them to two different companies, but I'd guess that it was the war-time paper shortage. Columbia had the material but no paper allotment to increase the number of comics it published so they arranged for a surrogate. This was not uncommon during WWII. For example, Vital published the first two issues of Plastic Man and Spirit for Quality.
I don't know which issues of Big Shot those extra stories are from -- I don't actually have those stories in my scan of Sparky. There were solo stories from around BS #94 floating around, though, as I recall.