Bro. Robert, yes these folks are all in the same correspondence groups.
In 1867, the Mountain Union Association was established, by members who had been excommunicated from the Senter and Mountain Primitive Baptist Associations in Ashe and Alleghany Co., NC and some from Grayson Co., VA. In 1868, the Primitive Association of Regular Baptist Association was formed from members who had withdrawn from the Roaring River District Primitive Baptist Association. These two associations were joined by an association that was formerly missionary, the Stony Fork Association in Watauga Co., NC. These were the original three bodies in the "Mountain Union Movement." In the 1890s the Primitive and the Mountain Union Associations divided for convenience, spawning the Little River Association (daughter of the Mountain Union); and the Mitchell's River and Blue Ridge Associations (daughters of the Primitive Association of Regular Baptists.) About this time the Stony Fork Association reverted to its former Missionary Baptist affiliations, so at this point in time ca. 1900 their bodies were:
Mountain Union (1867) - Ashe Co., NC & Grayson Co., VA
Primitive Association (1868) - Wilkes Co., NC
Mitchell's River Association (1890) - Surry Co., NC
Blue Ridge Association (ca. 1895) - Wilkes Co., NC
All four of these associations "corresponded" with one another, and all was well.
In 1910 the Mountain Union association had an unpleasant division and six churches withdrew forming the Macedonia Associaiton of Regular Baptists. This group (the Macedonia) wanted open communion, while the others insisted on close communion. The Macedonia Association ceased to exist after their 1916 session.
In 1935, the Mountain Union Association had another unpleasant division, and another group of churches left, and joined with some of the remnants of the old Macedonia Association. The issue at this time was women preachers and a general drift toward arminian doctrine on the part of those that were expelled at that time. Those churches that were expelled formed the Christian Unity Baptist Association. As you know this group was recognized for awhile as a completely distinct Baptist subdenomination.
In 1938 the Little River Association had an unpleasant division, resulting in the formation of the Union Association centered principally in Alleghany Co., NC. The Primitive Association in Wilkes County sided with the the Union Association and the Mountain Union Association sided with the Little River Association, so at this point there are two distinct fellowship groups. About this same time, the Mitchell's River Association was cut off from the others, and the Blue Ridge Association converted from "Union Baptist" to "Southern Baptist." So, at 1940 or so, their fellowship looked something like this
Group A
Mountain Union
Little River
Group B
Union
Primitive
Group C
Mitchell's River
In the mid-1940s the Little River Association fractured again (twice), so by 1950 there were three associations using the name "Little River Association of Regular Baptists." One of these bodies dissolved, and some members were reconciled back into one or the other of the Little River bodies, and a good number switched to become Primitive Baptists at this time.
During this time several people from this area had migrated to West Virginia to work in the coal mines, had established several churches there, formed their own association. In 1950, their fellowship lines looked like this:
Group A
Mountain Union
Little River#1
Little Valley
Group B
Union
Primitive
Group C
Mitchell's River
Group D
Little River #2
This Arrangement held until the late 1950s, when the Mountain Union Association divided over personalities, and the body divided "right down the middle." Two bodies came into being both claiming to be the Original Mountain Union Association of Regular Baptists, and the slightly larger body added the term "Original" to thier official name. The Little Vine Association in West Virginia divided about the same time, so in 1960 this arrangement came into being, and is still in existence.
Membership stats are educated guesses on my part, as I don't have any of their most recent minutes.
Group A:
Mountain Union - 12 churches - ca. 1000 members
Little River #1 - 6 churches - 500 members
Little Valley - 5 churches - 150 members
Group B
Union - 15 churches - 2000 members
Primitive - 20 churches - 2000 members
Original Mountain Union - 10 chuches - 1300 members
Little Vine (division from Little Valley) - 3 churches - 100 members
Group C
Mitchell's River - 5 churches - 400 members
Group D
Little River #2 - 5 churches - 400 members
So there are about 81 churches with about 7800 members. There are currently bodies in the following places:
Ashe, Alleghany, Watauga, Wilkes, and Surry Co., NC
Grayson, Smyth and Craig Co., VA
Monroe, Mercer and Greenbrier Co., WV
HArford and Cecil Co., MD
York and Chester Co., PA
Johnson Co., TN
About 5 years ago there was a movement afoot to put everything aside and reconcile, but it has so far come to naught.
As far as I know there is no real difference between these four fellowship groups.