Metal plate beneath Linothorakes or Spolades

[MeinPanzer]

"thought to come from Mesopotamia" sounds to me like a guess about an un-provenanced piece, and that appears to be all we have, and evidently Seyrig must have similarly supposed/guessed it to be 'Syrian' (Seleucid) if he discussed it under "Antiquites Syriennes".

No, he didn't. The article was titled "Les dieux armés et les Arabes en Syrie," and for the majority of it he concerns himself with Palmyran representations of armoured gods. He discusses the fact that cuirasses Greek in form but made of lamellar seem to appear only in Palmyran art, but then discusses whether this was a Palmyran invention, or whether it was adopted from the Parthians. He then states that "save one very isolated exception (this figurine, for which he cites in a footnote only Rostovtzeff), nothing is known of the cuirasses of the Parthians, but the lamellar cuirass appears to be foreign to the monuments of Doura" (p. 110). He makes no note of its findspot, but simply cites Rostovtzeff. There is no reason to take it as being from Syria.

It would appear that this piece, and its similar companion piece are 'origin unknown' and that the Louvre itself has gone with a 'best guess' of Syria.

Why is Syria a best guess? Considering that this guy's helmet is almost identical to that worn by the Seleucia cataphract figurine, evidently an Iranian type, there's a good chance that he is Parthian, and therefore the best guess would be Mesopotamia.

Incidently, if the tubular arm defences are like those of the Pergamum relief, then they are unlikely to be of metal, having no elbow joints and having a sort of 'folded under' construction in part - at a guess they will have been leather.....

We can't see both sides of these arm defences on the Pergamon reliefs, so we can't know if maybe segments were missing around the inside of the elbow region to facilitate the bending of the arm. That's really all it would take to allow such defences made of metal to be able to be functional