Perfins is the philatelic term for security perforations. It was once a common feature of stamps. To prevent employee theft, they gave stamps private perforations that have the company’s initials, or a simple design. As larger companies introduced metered post, the need for such security slowly vanished.
Here is an example of a Great Britain QV 2d from 1880 with the perfin JTM on eBay.
Perfins