I thank all the list members for their valuable comments.
It is true that the line of action of the normal reaction is displaced
towards the front edge and the distance between the lines of action
of the weight and the normal reaction can be shown to be ka/2.
Now my question is this. For the specified axis in the problem the
torque due to the weight and the normal reaction is the ONLY
torque that can reduce the angular momentum. But neither weight
nor the normal reaction are among the traditionally dissipative
forces. No doubt, the separation of their lines of action contains 'k'
and the magnitude of the torque is kmga/2. Can we say that a
dissipative torque can be due to nondissipative forces and a
dissipative separation?