I used the standard scope rail for the H&R, which included the hammer spur extension. Unfortunately, the extension was too chunky in size to install and clear the scope. I tried grinding it, but ended up with nothing usable afterwards.
What I'd like to do, per ideas from the FAQ, is drill a hole and mount an extension; instead of bolting something on. Haven't seen one yet that I can buy to do this. For now, my thumb clears enough to pull the hammer back as is.