I can't answer that explicitly, but it is possible that it only fits the Comet II.
That would be due to some unusual fit due to length of the threading on the spindle. Normally the insert fits against the face of the shoulder and not the end of the spindle. This allows slight variation in the distance from the face of the shoulder to the end of the spindle without that being critical. If the Comet II has too long a thread, a normal adapter may not seat properly and Technitool may feel that a longer insert may not be stable for spindles with a shorter thread.
On the other hand it could be just a marketing ploy...
Retired - when every day is Saturday (unless it's Sunday).