Originally Posted by squirtdad

sounds like your dura-ace AX again (case you have not guessed the AX group with all of it's aero design is an odd duck)

it seems they have and internal clamp..

guessing that you pop the dust cap and use long 6mm hex thru the stem to a bolt

Velo base is your friend

VeloBase.com - Component: Shimano HS-7300, Dura-Ace AX

came with a long 6mm allen key as clamping of both the bars and quill were performed internally.
The quill clamp bolt has 2 different threads so the tapered wedge was tightened or loosened depending which way it was turned and did not require hitting with a hammer to release. I know of 9,10,11,12, & 13cm. Shorter may have been made...