I don't know who Five Star Grading is; and as we know, there are a ton of illegitimate authentication services that are in the business of authenticating forgeries as genuine. CAS is a legitimate authentication service, though, that tries hard to get autographs right.
Steve Zarelli consults for them on space and aviation, so we know that's top-notch. The owner and chief authenticator, Mike Root, is honest. I haven't seen enough autographs authenticated by them, though, to know how reliable they are outside of space and aviation.
Why don't you call or email Mike Root, explain the situation to him, and see what he has to say? I doubt they guarantee authenticity, but I think you'll get a straight answer from him.