Filter data table by year

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: