I think the answer is that we can restrict it to a small number of possibilities. I will refer below to Branch 1 (containing Somersets 1 & 2) and Branch 2 (containing Somersets 3,4,5) - the structure of these will allow us to determine if particular lines could correspond to this. The following work assumes that
thepeerage.com is a complete listing of all male line descendants of the 5th Duke
The 5th Duke of Beaufort had a total of 7 sons who had sons themselves.
- The 6th Duke. The structure of his surviving male lines does not allow it to match either Branch 1 or 2. As noted, it is unlikely they would take part in case it did cause a disturbance to the Duke's or near kin's claim to the peerage.
- Lord Charles (1767 - 1831) - has 3 sons, but the male line of the second son (Villiers [1803-1855]) died out in 1953. This line cannot be Branch 2, but may be Branch 1.
- Lord Robert (1776 - 1842) - male line dies out in 1890
- Lord Arthur (1780 - 1816) - provided his grandson Fitzroy (b 1851) had no sons, then the male line is extinct.
- Lord William (1784 - 1851) - the structure of this family allows it be either Branch 1 or 2.
- Lord John (1787 - 1846) - male line dies out in 1915
- 1st Baron Raglan (1788-1855) - he has only 1 son, so this cannot be Branch 2. I believe the structure is also incompatible with Branch 1.
From the above, I believe Branch 1 represents descendants of Lord Charles and Branch 2 represents descendants of Lord William.
More work to follow