The 6c surcharges were made as a result of changes in postal rates. In April 1891, the postcard rate to UK and other countries in the UPU was set at 6c but this only lasted for 3 months before it was reduced to 3c. But also at this later date the normal overseas letter rate was reduced from 8c to 6c. The registration fee was also set at 6c. So these provisionals were well justified unlike so many other overprints from North Borneo. The 8c and the 10c stamps from the 1886 and 1888 issues were used. These were overprinted locally? with a series of errors. I am some way in completing the set with its errors but to achieve it fully is financially not an option.
The first stamp of this set, SG 54 used the 8c value from the 1886 "postage below" set. Apparently, only one sheet of 100 was overprinted and this accounts for its rarity and the high catalogue value. Beware there are various fakes in existence, some fairly recent. The best way is to compare the overprint with the same one on the cheaper stamps of this set. All the authentic examples that I have seen have full dark ink. The only error on this stamp is the large "S" located on stamp 19 or the 9th stamp on the second row. This is unique and it is the most expensive stamp from North Borneo. I think it was last publicly sold in 1965 by Robson Lowe Ltd, current whereabouts unknown. The most expensive stamp from North Borneo used to be the imperforate in between a horizontal pair of SG1. There are more than one example including an example in the Royal collection, I think and there is no reason for it to be more expensive.
This is SG 55, the 6c overprint on the 8c from the postage & revenue issue. The used examples show the Sandakan 19 bar, Labuan 9 bar and the Sandakan dotted modified 14 bar cancellations respectively.
These are the used and mint examples with the surcharge inverted. This error is located on stamp 44 of the sheet.
This a slightly defective example of the error with "cetns" located on stamp 27. Providence P K Cassels.
These are mint and used examples with a large "S" in the surcharge located on stamps 19 and 27 of the sheet of 100.
This is the surcharged 10c from the 1886 "postage below" issue with Kudat A cancel, mint copy and a most unusual still unexplained cancellation.
This is a mint example of the large "S" in the surcharge, again located on stamps 19 and 27. Mint examples of this set are usually without gum.
These are both mint examples of the surcharge on the 10c adhesive from the 1888 issue. That issue listed 2 shades, blue and dull blue. The surcharged set only listed the dull blue shade. The second stamp is the unlisted blue shade of this set which surprisingly is also not mentioned in the North Borneo handbook.
Lastly but not least, this is a mint copy of the error with a large "S", again with stamps 19 and 27. It has a fairly high catalogue value compared to used and is normally without gum. I have shown here almost £9000 of stamps according to the values using the SG 2017 catalogue but one can buy them in real life for a lot less.