informed note on the 'rusty mail' - those might be rings of copper. notice how regular they are - that also doesn't look like rust patterns either.
I saw several comments about that and added an EDT to the original post.
As mentioned there, the regularity of the pattern probably does mean decorative copper or bronze rings, but it was the general dinginess of the armour that made me think of rust before anything else - dull metalwork and an arming doublet like a doss-house mattress didn't prompt the thought of those discoloured patches being anything but rust. The person who could best tell us is the man wearing it, but I haven’t yet found a version of this photo with a caption explaining the way he looks - if there’s one somewhere, a link would be much appreciated.
I did a Google image search for “standard (chainmail)” and for “bishop’s mantle (chainmail)” - a “standard” is the collar-thing our chap is wearing, a “bishop’s mantle” is much wider and covers the tops of the shoulders. A couple of repros have bronze, brass or copper rings as decoration, but only along the neckline...
...or the hem (though if you go to the link, there's a hauberk with a chest-cross woven in zinc...)
“Internal” decorative patterns seem restricted to jewellery …
That is a seriously hard-wearing sweater!
Indo-Persian mail (when not involving splints or small plates) was much more likely than Western mail to have internal decorative weaving...
...including calligraphy...
...which, given the nature of Arabic script, must have been incredibly fiddly to do.