Sub Test_1()
    Dim rng As Range, cel As Range
    Dim startpoint As Long
    
    With Sheets("51batch")
        Set rng = .Range("A2", .Range("A" & Rows.Count).End(xlUp))
        For Each cel In rng
            startpoint = InStrRev(cel, "_") + 2
            'for test purposes write formula to column "K"
            cel.Offset(, 10) = "=Mid(" & cel.Address & "," & startpoint & ", 5)"
        Next cel
    End With
End Sub