But I just can invalidate your assumption using this most recent beta. Anything is possible. Apple has kindly broken the pattern. I don't mind they break it once again.https://9to5mac.com/2015/08/06/ios-9-beta-5-iphone-ipad-ipod-touch/
iOS 9.0 Beta 5 was the final beta and its build number was 13A4325c. Therefore, your theory is incorrect.
That was released roughly 5 weeks before the event. Therefore, being 3.5 weeks out, I am again certain this is the last build. There is no reason to have released two in one week.
Plus, the invisible "delete" button in music app is still there. No way they put it into official firmware unless they want to use it as Easter egg surprise.