Well, that is quite tricky question.
Scott created a branch from the 1.6.3 called dev. That branch was the alpha1 version. The alpha1 version lived for many months, receiving many commits. Today, one of those commits replaced the version string alpha1 with beta-1. So what commit (or point in time) do you mean by alpha1?
Think it this way beta-1 is the next step immediately after alpha1. So there won't be any differences between them right now (including the master branch).