A signed overpunch is a code used to store the sign of a number by changing the last digit. It is used in COBOL, especially when using EBCDIC. Its purpose is to save a character that would otherwise be used by the sign digit. The code is derived from the Hollerith Punched Card Code, where both a digit and a sign can be entered in the same card column.
Examples
10} is -100
45A is 451
Decimal points are usually implied and not explicitly stated in the text. Using numbers with two decimal digits:
1000} is -100.00
References
Signed overpunch Wikipedia(Text) CC BY-SA