Earlier catalogues also had the bag loops set into the leather.
The one from the 1920 catalogue had them. The saddle badge is in the earlier form, predating your saddle.
If you work through the catalogues on this page you can see the evolution of the B17's.
The width for the Standard did vary from the mid-1920s.
You can also see that the name badge guide provided at https://www.kurtkaminer.com/brooksbadges.html is not supported by the images in the Brooks catalogues.
But the B17 Champion Standard doesn't, in the catalogues provided on that site, have a keyhole cutout (other models do).

We then come back to the saddle in question, a B17 Champion Special. Not a Standard, and not in the catalogues.
I would be guided by the saddle badge design, the inclusion of the saddle bag loops, and the width corresponding to that of the B17 Standard. When these all met, then it would give you a year range for your Special.