Shield cover depicted on monument??

[Peroni]

Whilst recently viewing the casts of Trajan's Column at the EUR I spotted a couple of shields that appeared to have designs painted on the inner face. (Harder to see on the real monument)

The one on the far right is a wave pattern with an oval shape beneath each wave. The edge of the shield is also clearly defined. The soldier next to him also appeared to have a design too, but on closer insopection, the shield's edge was not defined, and it looks for all the world like a skin or cover that is stretched and tied behind the shield. I have highlighted the cover with a little colour to explain...

Anyone else noticed this before?



With shading and added tie string..