This post shows the back of the earliest bottle. It corresponds to the black script bottle on the left side of the first picture and was also found on the earliest 1939 red script bottle. No evidence of contents are shown anywhere on the bottle or the cap, which was likely blue. Notice that it's "different" which is in line with it being a new drink. Also it mentions how it can be used as "a drink a mixer". That market concept became prominent in late 1939 and into 1940.