Questions about Seleukid units.

[MeinPanzer]

In all fairness, the more Eastern appereances of mail have puzzled me slightly as well - I'm hardly an expert, but everything I've read has suggested it took fair a while before the stuff spread there in any quantities.
(One wonders if the proliferation of powerful bows had anything to do with it.)
On the other hand, I've always assumed the relevant EB folks have done their homework and debated the matter to death long ago anyway before implementing it, so...

I know of four instances of chainmail being depicted in a Seleucid context. This is one, describing the battle of Beith Zacharia in 162 BC:

And they divided the beasts into their formations and with each elephant they positioned a thousand men armoured in chain and bronze helmets on their heads.

Here it literally says "armoured in chains (or rings)."

Another is the description of the Daphnai parade in 167 BC in Polybius 30.25.3. He again mentions a picked force of 5000 men armed "in the Roman fashion" in mail.

The third instance is on the Pergamene weapons reliefs, which do show two mail cuirasses. One is positioned beside many thureoi, spears, and a carnyx, implying that it is Galatian; the other is identical in form and is shown behind what looks to be a Celtic-style sword, so it too is probably Galatian.

The final instance is the famous painted stele of Salmas of Adada from Sidon in the 2nd C. BC. It shows a man with a bronze helmet, a thureos, a spear, and what appears to be an iron mail cuirass. It is unclear whether the Sidon stelai show Ptolemaic or Seleucid mercenaries, and of the many found, Salmas is the only one wearing mail.

The only two instances of mail being worn by a significant amount of men in the Seleucid army refer to the same unit (imitation legionaries) which existed for what seems to be around half a century at most.