Tawau paquebot

A paquebot cover which was probably posted while the ship was on its way to Tawau. An air mail envelope was used and presumably this then continued  its journey to Sibu by air.
It took me a bit of time to understand the workings of paquebot mail. In practice, there are 2 countries whose stamps are allowed for postage on any ship. Firstly, the stamps of the country under whose flag the ship is sailing. This is relevant when the letter was posted in international waters. An example would be the Labuan paquebot cover as in the March posting using a GB stamp for a British registered ship.
Secondly, once the ship is within territorial waters, then stamps of that country would be valid as in the various examples which has been shown so far.
The situation gets more confused with letters which were posted at the harbour direct onto the mail launch. Normally, there is a surcharge. North Borneo had a system where these letters were given a "late letter" bar cancellation. This was not used by most countries.
Sarawak has many rivers which were used instead of roads for the transport of goods and mail. That is why there was a profusion of paquebot cancellations on Sarawak stamps. Most of these were Singapore paquebot markings. These cancellations were made on arrival in Singapore itself and also on board certain designated shipping vessels.
Mail from Sarawak and Brunei were known to be carried on ships bound for Labuan and cancelled there. The former is very rare (I say this now and at some time, a whole lot would suddenly turned up together just like London buses!) and the latter is also scarce.
The term "loose letter" can also be applied to paquebot items. Normal mail are carried in sealed bags. Mail posted on board ship are "loose" as they tend to be a few items handled separately. The term "ship letter" applied to postal letters carried on private shipping and would have a cancellation with the name of the ship.