It has been said that philately is the art of uncovering mistakes and imperfections.
This variety on the North Borneo 1954 10c stamp (SG378) was first written about by a prominent member of the Sarawak Specialist Society in the Sarawak Journal in 1972. It was later positioned as the top right stamp on a sheet, R1/10. To date, this variety has not been noticed in the very similar 1950 George V 10c stamp.
It was brought to my attention a few months ago by the June edition of Gibbons Stamp monthly. The catalogue editor has strongly implied that this variety would be included in the 2014 edition of the Stanley Gibbons catalogue after 40 years!
So how uncommon is this stamp? To date, I have only seen 4 copies of this variety after perusing much in excess of a hundred over copies of this stamp. Two of the stamps are shown above, a mint and a used example, between which is the normal for comparison. I also had sight of the part mint sheet with its position in the possession of a fellow member of the society. The variety is shown as an "extra chimney" on the sawmill at the top of the design. There is at least one example of this stamp on cover.