In 1922, Brunei joined the Straits Settlements states exhibiting its products in the Borneo-Malaya Expo which was held in Singapore. It is not known whether Brunei actually exhibit its products and who actually went to the expo. But what is known is that during the expo, all the Malaya states plus Singapore, North Borneo (present day Sabah), Sarawak and Brunei all produced overprinted stamps - stamps with the words Malaya Borneo Exhibition. In Brunei's case, it was Malaya Borneo Exhibition 1922. Most of the other states just overprinted with the words 'Malaya Borneo Exhibition'.
These stamps were not in the earlier form accepted as new stamps as they are old stamps with the overprints. But over time, these have been accepted as a special print and are now recognised. I have been trying to collect the other staes ever since I completed the Brunei set a few months earlier. The other day I was able to buy additional stamps for the Kedah set. I am only short of the $1 stamp for the Kedah state. If you happen to have that particular stamp, please let me know.