This tray is identified in the "Great American Glass from the Roaring 20s and the Depression Era" by Measell and Wiggins (GAG books) as a #9 tray. Another shape handle is also documented on as #9 in Craig's Indiana books.
The former CHS editor had both this tray and the other shape handle identified as #9s. The GAG books are known to have some misidentifications so let's look for more info on this one.
Craig says that All of those CHSs are Indiana's #9 pattern. They had four CHSs in the line altogether. Two were round at 10" and 11", one was octagonal at 10" and then there was the small 6" round butter ball tray. They had many decorations, both by Indiana and outside companies.
Rainbow originally was only a decorating company so a Rainbow label does not always mean they made the glass.
