All the shields would have been wood. Officers and standard-bearers sometimes used smaller shields according to bas-relief evidence, one presumes for mobility when moving around the ranks.
I guess I have the roman fantasy of the metallic square scutum(?), when I did a quick read up was was made with quality layered wood and strategically placed metal trimmings and a metal boss at its center that served both in offence and defence
First thing I thought when I saw the centurion's shield was if it was an auxilia's shield, but it makes sense for them to have lighter equipment if they were meant to act as command officers and to inspire confidence in the ranks at crucial times
Interesting read, seems it was different according to prevailing doctrines at the time
I find the discussion around the centurions on the edges - thinking how it could afford better mobility, situational awareness and adaptability without throwing the formation too much out of step if legions were required to lock shields and when charging was required allowed the centurion enough space to lead it if necessary