I have found that the HX =H123 keyblank due to H123 being too long. These are typically used with master keys that pass through Hagers H1,H2,H3 different keyways.
From my experience I can tell you that location is where the manufacturer would stamp the keyway so it was my suspicion that is what they would stamp when they were using the all-section key blank which is their h123 designation.
Additional information on the hager keyways can be reviewed at the document as seen here:
Download File
The manufacturer is correct and when they state that they would use the all section blank when the sheer size of the required practical key changes exceeds what can be accomplished through a single key way they would then implement one or two of the additional key ways within the multiplex system.
You want to be very sure and very careful to not cut change keys under a master on the all section blank because you're very likely producing a key blank that will work in other areas that it was not intended to work in. This would be key interchange and to be avoided.