For a long time, nobody in the world seemed to collect Thailand revenue stamps with security markings besides myself. They are seldom offered on eBay
but I was always the only bidder, till recently.
These security markings had the same main purpose as security markings on postal stamps, preventing pilfering by employees.
This was specially important for revenue stamps which had many times a face value much higher than postage stamps. In addition in Thailand,
unused revenue stamps could be returned to the revenue office.
These markings started in Thailand around 1954/ 1955 and only on stamps used by foreign owned banks. The latest observed markings are from 1998.
Below I show some security markings of the H.&S.B.C., the Hong Kong Shanghai Banking Corporation.
If there is interest in this subject I can post more different banks