The Lost Tribe of Joseph is 2 modern nations...
Joseph's son Ephraim is the United Kingdom.
Joseph's son Manasseh is the United States.
Joseph is an interesting tribal name because alone Joseph was not a landed tribe of ancient Israel. This is because he was written out of the will by Jacob, and his 2 sons, Ephraim and Manasseh replaced him.
The name Joseph shows up in the seal date system, though, and it identifies the March 1941 signing of the Lend Lease Treaty that established the basis for the United States to fund the United Kingdom in World War II.
Manasseh already has a seal date, in 1776, and it identifies the tribe of Manasseh as the modern United States.
By process of elimination, this leaves the tribe of Ephraim as the modern United Kingdom.
There are several interesting components to this structure that are important to note. Manasseh is the son who inherits the double portion birthright promise all the way from Abraham. This double portion aspect of Manasseh is being used on this date too, since Manasseh is in effect party to 2 of the seal dates.
Most of the rest of the tribes also carry their own native tongues. In this case Joseph, the English speaking world, breaks into 2 tribes with the same language, which is another interesting feature.
Note also that both tribes have essentially 50 base component "states."
In the case of the United States those 50 are not international sovereign governments, while in the case of Ephraim they are.
Jacob's prophecy about Ephriam:
15And Jacob blessed Joseph his son saying, The god before whom my fathers Abraham and Isaac walked righteously, the god who has supplied my needs from my youth to this day,
16the king who has delivered me from all evil, bless the lads; and let them bear my name and the names of my fathers, Abraham and Isaac; and let them grow and multiply in the middle of the land.
17And when Joseph saw that his father laid his right hand on the head of Ephraim, it displeased him; and he held up his father's hand, to remove it from Ephraim's head to Manasseh's head.
18And Joseph said to his father, Not so, my father; for this is the firstborn; put your right hand on his head.
19But his father refused, and said, I know it, my son, I know it; he also will become a people, and he also will be great, but his younger brother will be greater than he, and his seed will become a multitude of nations.
20And he blessed them that day saying, By you Israel will bless, and they will say, May god make you as Ephraim and as Manasseh; and thus he set Ephraim before Manasseh.
The blessing given to Ephraim and Manasseh upset their father Joseph because Jacob appeared to be changing the natural birthright promises.
The passage suggests that Manasseh would inherit Abraham's Great Nation promise while Ephraim would become a community of nations.
Of course this is how it worked out, the United Kingdom is a nation, but also a network of related nations including Canada, Australia, New Zealand and many others.