Find the missing number in Sequency

MightyRabbit 
The solution you linked to is in DAX, and that will work. This solution is in Power Query. Here is the result:

Here is what I did:

1. Found the series base - F65, F66, etc.
2. Extracted the numerical portion - 1234 for example
3. Grouped by the series base and added all data to nested tables
4. Added a list of missing numbers by getting the min/max of each series base and creating a full list of those numbers, then removing those that already existed. So for F65 it returned a list of 1236 and 1237
5. Expanded those missing numbers and added the series base back to them, then added a column that simply said "Missing"
6. Removed unnecessary columns
7. Appended the missing numbers to the original table.

Here is the code:

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WcjMzNTQyNlGK1YGxTZHYFkhsSyjbDKjcFIlthsQ2R2ID1ccCAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Series = _t]),
    #"Added Series Base" = Table.AddColumn(Source, "Series Base", each Text.Start([Series],3), type text),
    #"Added Numbers" = Table.AddColumn(#"Added Series Base", "Numbers", each Number.From(Text.AfterDelimiter([Series], [Series Base])), Int64.Type),
    #"Grouped Rows" = 
        Table.Group(
            #"Added Numbers", 
            {"Series Base"}, 
            {
                {
                    "All Rows", each _,  
                    type table [Series=nullable text, Series Base=text, Numbers=number]
                }
            }
        ),
    #"Added Missing Numbers" = 
        Table.AddColumn(
            #"Grouped Rows", 
            "Missing Numbers", 
            each 
                let
                    varAllNumbers = {List.Min([All Rows][Numbers])..List.Max([All Rows][Numbers])},
                    varExistingNumbers = [All Rows][Numbers]
                in
                List.Difference(varAllNumbers, varExistingNumbers)
            ),
    #"Expanded Missing Numbers" = Table.ExpandListColumn(#"Added Missing Numbers", "Missing Numbers"),
    #"Added Full Series" = Table.AddColumn(#"Expanded Missing Numbers", "Full Series", each [Series Base] & Text.From([Missing Numbers])),
    #"Added Is Missing" = Table.AddColumn(#"Added Full Series", "Is Missing", each "Missing"),
    #"Removed Other Columns" = Table.SelectColumns(#"Added Is Missing",{"Full Series", "Is Missing"}),
    #"Renamed Columns" = Table.RenameColumns(#"Removed Other Columns",{{"Full Series", "Series"}}),
    #"Appended Query" = Table.Combine({Source, #"Renamed Columns"}),
    #"Changed Type" = Table.TransformColumnTypes(#"Appended Query",{{"Series", type text}, {"Is Missing", type text}})
in
    #"Changed Type"

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