Brunei covers and especially the commercial ones, should be much more uncommon than those from Sarawak and North Borneo. However, there is no price premium due to less demand and fewer collectors of this country's philately.
This 1922 cover to Kansas, USA probably has the right rate of 12c+12c for a registered item even though this was also the 1923 rate. No postal rate changes for Brunei between 1908 and 1922 were published. It has the registration cachet R1 and Proud D5. The first 2 digits of this cancellation is very often missing contrary to what is shown by Proud. It also looks peculiar as the year digits seems to be pushed to the right side.
The back stamps tell a clear story of the journey. It went to Labuan and then Singapore before making its way to USA. There is a nice clear Labuan D10 with an A time code. It took a total of 46 days to reach its destination.
Our well known friend, Paul Kiderlen, was sent this cover. But this not his usual Ulm address in Germany. Instead it was sent to Schwabish Hall (the short form Hall was used on the cover) in Baden-Wurttemberg, Germany. Almost all covers to him would have been addressed to him in Ulm in the same state over 120km away. Kiderlen was very active between 1890 and 1910. This is actually a rare Kiderlen item during a time when he might not have been active dealing in postal history.
This is a large envelope bearing the rate 12c first oz + 6c for an additional oz + 12c registration fee. It also has Proud D5 and R1. There is a high probability that this was a commercial item enclosing a 2 oz stack of documents. As normal, it went to Labuan and then Singapore before going to Germany probably via Brindisi in Italy. The Labuan cancellation is a D9 with smaller letters for Labuan as compared to D10 or D11. The receiving chop for Schwabish Hall is very likeable.
A desirable but slightly defective Brunei MBE 1922 cover. The rate of 5c+12c was probably underpaid by 1c even though we have no idea what the rates were in 1922. It did not attract a T mark and a postage due which was a pity. It also has Proud D5 and R1. There is a Labuan transit cancel with D10 with a C code. The Singapore arrival cancel is unclear but from the other 2 covers, we know its journey would have taken about 5 days.