These were used to mark mail carried on a series of well documented flights around North Borneo in June and July 1930. These North Borneo RAF survey flights of the coast were carried out by two Flying Boats of the 205 Squadron.
The majority of covers were philatelic but are nonetheless very sought after. They constituted a specialised area of collecting for North Borneo.
Update: Over the past 12 months, I have been able to examine a number of of covers posted from the various towns with the cachet. The conclusion is, after discussion with colleagues and despite what has been written, the same cachet or cachets were used on all the flights. I think there were one or possibly two handstamps which were carried on the planes themselves. It was likely that they were constructed locally with either rubber or a local hard wood. I favour the latter due to the clarity of the cancel above and also has to be durable enough to survive the war. Rubber would have degraded after a number of years.
I have yet to see a non philatelic cover from this series.
The old post office in Jesselton/Kota Kinabalu is situated on the northern end of Gaya Street is now the local tourist office.
I have included this nice philatelic item bought for very little money. It has an attractive fancy type airmail cancellation which can be found on some USA and Philippines covers. In addition to the airmail hand stamp, there is also a Houston Texas cancellation in red violet. It transited in Galveston on its way to Chicago.
There is a series of these philatelic Java-Borneo airmail covers posted from Balikpapan. This one has the appropriate stamp.