Hello,
First off, thanks for having such an awesome reference site...I use it daily.
I was able to obtain a very rare 91-92 pro set prototype blank back from the personal collection of Steve Taft. He obtained this along with dozens of the regular backed prototypes back in the early 90's and thought it should be included in the checklist as BGS has deemed them authentic. Authentic because they were hand cut. There is only 2 known in existence at this time. I'd be happy to share any additional information if needed.
Thanks