I find these curious piece of brestplate in the plates of "German Medieval Armies 1300-1500", but I didn't find it anywhere else. Where and when this brestplate came from? What is the purpose of this chains?

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And this Eastern Europe's lamellar or scale armor, which I also found it here

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Picture Source: http://www.warfare.altervista.org/WRG/Feudal-...an-12C.htm

This could actually have been made of steel? Where and how long It became common?

cant say much for the eastern European armour, but the other one, the oval shape for the breastplate is a bit odd (though I think I've seen it in art) but the chains are absolutely right. From what I've read, they were used to attach the dagger, a great-helm, etc. through the toggle-like bars at the end of the chains, so they couldn't be lost. No idea if it works, though.

An example of that sort was found in a dig about 10 years ago at Hirchstein Castle, in Germany. I think its now in a museum, Ingolstadt ,or Innsbruck, I think. ... something beginning with "I", at least.

I believe that Capwell might have written about it at some point.

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example from Hirchstein castle, photo from Herman Historica auctions.

Bamberg Cathedral, c.1370:
http://effigiesandbrasses.com/742/4746/
http://effigiesandbrasses.com/743/1021/

The chains are, as noted, used for retention of both weapons and great helm. They can be attached to pairs of plates or aketons, numbering from 1-4. They appear in a number of 14th century inventories, sometimes being made of silver. The earliest instances of their use dates to the second decade of the 14th c..

As for the scale armor, it looks to be based upon the Cathedral of St. Lawrence, West Portal, Trogir Croatia carvings. The figures are near the top of the arch, and I haven't found a good photo, yet.

Sometimes the chain was attached to the saddle. One tactic was to charge while wearing the great helm and then, after entering melee, this helmet was removed and the smaller helmet underneath was then relied upon. The chain was to make it easier to retrieve the helmet afterwards.

The retention chains were used in England, France, and Italy as well, though they seemed to find more lasting popularity in German lands. There might be one or two examples from the first decade of the 15th century, but the chains are primarily a 14th century phenomenon.
http://effigiesandbrasses.com/search/?tags=%22chains%22

1311 Inventory of John fitz Marmaduke, Lord of Horden
j gaimbeson rubeum cum tribus cathenis argenteis l s.
1 red gambeson with three silver chains, 50s.

1313 Acquittance of goods seized from the captive, Piers Gaveston, Earl of Cornwall
Item, En un autre Coffre, une Peire de Plates, enclouez & garniz d'Argent, od quatre Cheynes d'Argent, coverz dun Drap de velvet vermail, besaunte d'Or.
Item, in another coffer, one pair of plates, nailed and garnished with silver, with four silver chains, covered in cloth of vermillion velvet, bezanty gold.

1327, England: http://manuscriptminiatures.com/5715/21385/

Stop using Osprey. Most of the illustrations, especially the colour plates, are speculative fantasies. The books have some photos of extant artefacts that can be useful but not much else. Some Osprey books are very good such as the ones writtern by Graham Sumner, but they are the rare exception. Stick to books that employ footnotes and citations so you can confirm the veracity of the author's statements.