as we have split the input table into different tables we can go to each table and retrieve the Beleg numbers as the same we can process a datatable e.g. for each row.
In case that you are only interested on the list of Beleg numbers we can also creatre a lookup dictionary where key is the year and value a string list with the corresponding beleg numbers
Assign activity:
dictYearBelegLK | Dictionary (Of String, List(Of String)) =
(From d in YourInputDT.AsEnumerable()
Group d by k=d("GJ").toString.Trim into grp=Group
Let bl = grp.Select(Function (b) b("Beleg").toString.Trim).toList
Select t = Tuple.Create(k, bl)).toDictionary(Function (x) x.Item1, Function (x) x.Item2)
Also have a look here: