When errors were discovered on Canadian notes, the printing companies came up with the idea to make up "replacement" notes or sheets that would be inserted to replace the error sheet. They first started to use an asterisk "*" to precede the prefix, then in the later years they used an "X" in the 3rd digit of the prefix. However, the "X" in the prefix should not be confused with an "X" as the 2nd letter of a 3 letter prefix, being a test note. When a sheet of errors were discovered the employees would simply insert a "replacement" sheet. It appears that some of the replacement prefixes were very common, and with others perhaps only a couple of sheets printed.
To read a wonderful explanation about replacement notes written by Steve Bell, click here
These are examples of replacement notes of the 1954 modified notes. Asterisks "*" are used to identify these as replacements.
These are examples of replacement notes of the multi-coloured series notes. Asterisks "*" are used and also an "X". Both examples are shown here.
These have the "X" as the third letter.
However, test notes use an "X" as the second letter and are used to test new paper or ink. These are not replacement notes.
In the 1979 issue, there were no letters were used as a prefix of the serial number. In order to depict the printer of the notes, numbers were used. With the $5 notes the number 310 meant that the note was a replacement. With the $20 notes 516 and 510 were used.
In the new Bird series an "X" is also used in the third letter.