I had the same question, so called Diamond/Masterbrand customer service. They said they were the same. If you want to compare even further, Diamond is Schrock at Menards, and Thomasville at Home Depot.

When I was buying and researching a few years ago, the construction specs for Diamond and Thomasville were not exactly the same. The backs of the Diamond were thicker than the Thomasville for one thing. I can't remember off hand if there were other differences. Looking at Thomasville and Diamond catalogs, they look exactly alike.