British Kut PERFINS HARIASTAMP QEII 5s val I cannot see a postmark but some ink is visible on the edge of the stamp. It must have been used.
The PERFINS are the letters with the holes in the stamps.
Please view the photos carefully.
I can make out S, A, it could be the SOUTH AFRICAN BANK CORPORATION (??) But I have no further knowledge of this, sorry....
This will enhance any mature collection.