[gonzo]

I’d have to say it’s unique unless whoever has the Hindu Johnson also has a similar group. Some other HoFs -could- exist with Hindu backs (anyone with the same backs as Johnson, so Young or Jennings), but I haven’t heard them reported as existing yet. A Drum Wheat could exist but has not been reported yet.

If you count “blank back” as a back, there’s a Speaker and a Cobb, and I believe a Mathewson, though I’ve never seen a scan of the latter. There’s also the McGraw scrap from Leon’s collection that Heritage auctioned a few years back. And I believe that a Collins Mouth Closed probably exists, but I’ve never heard one reported.

You might throw Piedmont 42 into your list as well. A bunch of Hall of Famers are available with the P42 back.