Thank you, gentlemen, for the exhaustive explanations. I always admired the
Roman art. Apparently, the engravers used a particular language and "specific" means to depict objects in the best way, using the knowledge available. "Specific" - to avoid difficulties with two-dimensional presentation of three-dimensional objects.
The
side views of sitting persons in the above examples allow us to imagine easily the remaining details (such as the second pair of legs, e.g., hidden in the shadow). Possibly, the depiction of a curule chair was a too compicated problem. It was solved just by rotating the low
part (to show X-shape) which is hardly to justify. Of course, the reputation of the ancient artists could be saved if we interprete the chair as a "curule bisellium" but, unfortunately, there are no confirmations that a such one did exist.