If the stern had transoms above the wing transom, they would no longer be affixed to the sternpost.
From the en.wikipedia.org
Divers, nevertheless, recovered a sternpost and rudder, but the tiller and everything else above was missing.
From the usatoday.com
Scheme of a sternpost-mounted medieval rudder.
From the en.wikipedia.org
Brunel reported in June 1857 that once the screw, screw shaft and sternpost had been installed the ship would be ready for launching.
From the en.wikipedia.org
Advances in shipbuilding included the multi-masted ships with lateen sails, the sternpost-mounted rudder and the skeleton-first hull construction.
From the en.wikipedia.org
Chinese stern rudders also featured tackles, but, unlike their medieval and Arab counterparts, had no sternpost to which to attach them.
From the en.wikipedia.org
The stern frame provided the foundational structure of the transom stern, and was composed of the sternpost, wing transom, and fashion piece.
From the en.wikipedia.org
There is no evidence to support the contention that the sternpost-mounted rudder came from China, and no need to call on exterior sources for its introduction into the Mediterranean.
From the en.wikipedia.org
The stern is the rear or aft-most part of a ship or boat, technically defined as the area built up over the sternpost, extending upwards from the counter rail to the taffrail.
From the en.wikipedia.org
More examples
(nautical) the principal upright timber at the stern of a vessel
A sternpost is the upright structural member or post at the stern of a (generally wooden) ship or a boat, to which is attached the transoms and the rearmost left corner part of the stern. ...
A long straight piece of timber erected on the end of the keel to form the rear end of a ship and to support the rudder.
An upright member rising from the after end of a keel; a rudderpost or propeller post.
Main member at stern of a ship extending from keel to deck