Old HD580 with brass ring in the magnet assembly and black paper filter. Later versions have black synthetic (nylon?) and fine metal weave filter respectively. Some listeners claim sonic differences. This filter balances the ratio of direct and indirect (reflected, delayed) sound and has a crucial influence on airiness and clarity.
One of the reasons Sennheiser changed material was that it was affected by humidity. As you can see the paper filter is sagging to the right. Apparently there were probelms keeping the synthetic weave stretched, too, so Sennheiser finally settled on metal:
"A specially developed damping element, made from fine acoustic metal mesh, ensures a precise damping over the entire diaphragm surface, highly constant in all climates."
My HD580 has a wonderfully balanced sound through the frequency spectrum, with fine stereo depth and width. There is a hint of dynamic politeness though, that I need to define with further amp trials.