The 2, 4 and 10c provisionals of 1916

The reason for these overprints may be due to supplies of these values running low as well as a further consignment lost at sea due to enemy action as part of WW1. The surcharges were carried out at the Government Printing Office at Sandakan probably around February 1916.
The variety inverted "S" occurred on stamps 20, 65 and 70 on a sheet of a hundred. 25 stamps were surcharged at a time. The stamp on the right has the error. But also note the thinner numeral and letters. The letter "e" also seems to form a complete circle.

Here we have a used pair on piece with the one of the pair with the error, almost certainly cancelled by favour. It has been recorded that a rogue employee of the printing office was using the error surcharge on single stamps on purpose. Therefore stamps in pair with the normal surcharge are most likely to be genuine whereas single stamps can be suspect. The 2c stamp with the thinner surcharge could well be one of these.
 

The normal p15 stamp is actually more uncommon than a p14 inverted "S" stamp. The p15 also exist with inverted "S" but it must be quite hard to find. The surcharge of the stamp on the right also seems to be a lighter shade. It has been written that that the 4c and 10c surcharges were carried out in both carmine or vermillion but the colour differences are probably not that clear cut.

Here we have a mint block of 4 with the top right stamp having the inverted "S". Lastly, there is also a variety of the surcharge where the letter "S" has fallen off and was later inserted by hand. This exist for both the 4c and 10c surcharges. They are very rare and I have yet to have sight of one. 

But the most valuable stamp of the group is the 2c double surcharge. I have seen one stamp so far but it looks more like an offset rather than a true double.