Actually, a worldwide square circle postmark collection with associated postal history would make a very worthwhile and interesting exhibit. From British Borneo, there were only two postal cancellations of this type and they both originated from North Borneo.
The very first cancellation of this form was introduced in late 1879 in the United Kingdom as an alternative to the more cumbersome duplex cancellers. The use of this type of cancellation then spread throughout the Commonwealth and the rest of the world. A square circle cancellation can be described as a cross between a cds and a killer bar cancel.
This is number 8(as seen above) in the series of postcards by Phillippe B. Funk of Sandakan. It has a view of the Recreation Club and the Chinese temple of Sam Sing Kung, the oldest standing structure in Sandakan. The temple was completed in 1887.
We have the Sandakan square cancel D11 (1904-1915). The date here is 1 MAR 1909. The habit of placing the adhesive on the picture side is to facilitate display of both the stamp and card. I think this custom originated from France.
On the back, we have Funk signing off as F Philippe with CCC 3860 which was his number for the Cosmopolitan Correspondence Club in Milwaukee in contrast to number 2724 for his business colleague Ha Buey Hon of Sarawak. Funk himself probably did not collect used postcards for long as I have seen a message in a sister postcard that asked for unused cards of beauties and actresses. He was obviously selling them in his shop.