Octagonal templon column and capital, 12-13th century AD . From the Wikipedia: A templon (from Greek τέμπλον meaning "temple", plural templa) is a feature of Byzantine churches consisting of a barrier separating the nave from the sacraments at the altar. The solid templon first appeared in Christian churches around the 5th century and is still found in many Eastern Christian churches. Initially it was a low barrier probably not much different from the altar rails of many Western churches. It eventually evolved into the modern iconostasis.
In 2007 I ran into a special exhibition dedicated to Byzantium. I took pictures, maybe some objects were on loan from elsewhere. This might be one.