Babylonian Jewry .....and
[Josephus, Antiquities of the Jews, Book XI, Chapter V, Paragraph 2]"When Esdras had received this epistle, he was very joyful, and began to worship God, and confessed that he had been the cause of the king's great favor to him, and that for the same reason he gave all the thanks to God. So he read the epistle at Babylon to those Jews that were there; but he kept the epistle itself, and sent a copy of it to all those of his own nation that were in Media. And when these Jews had understood what piety the king had towards God, and what kindness he had for Esdras, they were all greatly pleased; nay, many of them took their effects with them, and came to Babylon, as very desirous of going down to Jerusalem; but then the entire body of the people of Israel remained in that country; wherefore there are but two tribes in Asia and Europe subject to the Romans, while the ten tribes are beyond Euphrates till now, and are an immense multitude, and not to be estimated by numbers." to be found here.
You posted Josephus: "...wherefore there are but two tribes in Asia and Europe subject to the Romans, while the ten tribes are beyond Euphrates till now, and are an immense multitude, and not to be estimated by numbers."
Hmmmmm...Josephus, AJ, 11.5.2. Sounds like fancy. The ten tribes disappeared and were not known after the end of the Babylonian-Assyrian eras.
Most likely Josephus was not relying on info he knew first hand, but upon common apocrypha of the day. In THE TESTAMENT OF MOSES, we find: "Then some of the tribes will go up and come to their appointed place, once again surround the place with walls. Two tribes will continue on in the way appointed for them and the ten tribes will be fruitful and increase among the Gentiles during their time of captivity."