I have done my due diligence and I haven't found a way to determine what key number I need. I have, however, removed one of the lock barrels in the hope that I could find the right key or get one made.
All I can say at the moment is that #26 and #27 are not the right keys, although both of them can be inserted fully into the barrel, so they are the correct blank. I've been to my local locksmith and he couldn't find the correct blank in stock.
Would anyone on this forum be willing to try your H-B keys in my lock barrel if I send it to you?