The 1899 8c postcard was issued for use to non union countries after North Borneo joined UPU in 1891. But its use was largely obsolete when the foreign UPU postcard rate was set at 4c in 1895 and the cards were surcharged 4c. Surplus supplies were already surcharged 1c in 1892 for local use.
Some of the cards were printed in error with inverted "8" at the left hand corners as shown. The mint copies are uncommon but not rare.
Now and again a used copy would come up at an auction. They are a bit more difficult. This one was bought at a society auction auction a few years ago.
The 1895 4c surcharged cards with this error are rare. This is the third copy that I know of with this error on a 4c surcharged card. Altogether there are 2 used and 1 mint cards on record with this error. In addition, there is also one mint 1892 1c surcharged card recorded with this inverted "8". This PSC was also surcharged 4c in 1896 for use in Labuan and the error on this card awaits discovery.