Metal plate beneath Linothorakes or Spolades

[Lysimachos]

While this is true, this is only the case for heavy cavalrymen. That armour - and similar armour from Takht-i Sangin - belongs to cataphracts, and shows close affinities to Central Asian heavy armour (and contemporary Parthian armour, in particular). This makes sense, considering that it was directly adopted from Central Asian peoples, and it is very distinctive, being made primarily of iron scales and hoops. Iconographic sources, however, make clear that beyond this armament in use among heavy cavalrymen, Graeco-Bactrian armament was purely Hellenistic, including T&Y and muscled cuirasses. Considering that this fragment is made up of two large plates riveted together, the only real candidate for what this could be if it were armour would be a Greek style cuirass.

I generally agree, but the shoulder pieces from Ai Khanoum are made up of smaller scales and larger plates. I'm not aware of any iconography that shows cataphract armor (either Central Asian, or Greek) with shoulder pieces, but it seems to me these could be part of a metal T&Y cuirass used by a Graeco-Bactrian cataphract. Just because we have no full depiction of such a suit of armor does not mean it wouldn't exist. The combination of a Greek and Central Asian elements is certainly not unknown in cataphract armor, for example in this statue with Greek-style cuirass (muscled in this case) and hooped limb armor from Syria from the Seleucid or Parthian era in the Louvre:
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As I noted before, Alexander's cuirass from the mosaic looks to me like a combination of metal plates and scales. This combined with the existence of plate/scale armor fragments from Ai Khanoum could mean that such metal T&Y cuirasses were used in the east. As long as we are in the realm of wild speculation, that is

I do not think, however, that this is armour, as its general form is not really indicative of any kind of armour that I am aware of. Without further information contextualizing it (i.e. if many small fragments of sheet bronze were found around it but not published, or other arms and armour were found nearby), I would just identify it as a piece of sheet bronze of unknown use.

Yeah, Nikonorov doesn't really say much about the find to give us an idea of why he thinks it is armor. I should mention that descriptions of the Ai Khanoum armor I have used are also from a work by Nikonorov (Armies of Bactria), and I don't know how much he got from the original report and how much is speculation on his part.

BTW, I have never heard of armor from Takht-i Sangin, did you mean Old Nisa?