Replacement Banknote

A replacement banknote is a banknote that is printed to replace a faulty one and is used as a control mechanism for governments or monetary authorities to know the exact number of banknotes being printed. As quality control finds defective notes in the printing process after the serial number has been overprinted, they are taken out with their serial number written down and replaced with another banknote printed specifically for this purpose, so that the amount of banknote being printed stays the same in each production batch. This saves time and money compared to re-printing exactly the same serial number that was used before. It is rare that the replacement banknote has the same serial number as the original faulty one. A replacement note will have its own serial numbering system that separates it from the normal numbering system.

Some examples of replacement banknote marker by countries:

The United States uses and Australia did use (stopped in 1972) the "*" in the serial number to mark a replacement banknote. These are known as "Star Notes".
Argentina uses "R" serial number to mark replacement banknotes.
Bahama uses "Z" serial number to mark replacement banknotes.
Hong Kong and Mongolia uses "ZZ" serial number to mark replacement banknotes.
Singapore uses "Z/1" serial number to mark replacement banknotes.
Indonesia uses "X" serial number to mark replacement banknotes.
Zambia uses "X3" serial number to mark replacement polymer banknotes.
Thailand uses "5 OS" serial number to mark replacement polymer banknotes.

The list goes on and different countries will have their own numbering / marking system on their own banknotes. There is no guaranteed way to know the rarity of replacement banknotes as they are only introduced to replace broken ones. Some banknote collectors go after these rather unique banknote as their specialty. Both paper and polymer replacement notes exist as this control mechanism.