These stamps are supposed to have identical overprint but they don't. The one on the left is mint with the overprint appears thicker than the one on the right. There is also other inelligible violet overprint at the back of the second line overprint. The stamp on the right is used, with visible Sandakan double ring cds. The overprints are thinner compared to the left one.

The stamp on the left is claimed to have a certificate of authenticity but the appearance is very similar to the ones known for being fake (Compare the overprint here). The one on the right on the other hand, appeared in ebay some weeks ago with an opening bid of only GBP 0.50!