Japanse forces landed in North Borneo on 15 December 1941 and Brunei was occupied by 6 January 1942.
Current stamps continued to be used until October 1942 after which the stamps were only valid for postage if they had been handstamped in Japanese. The handstamped overprint translates as "Imperial Japanese Government".
Five values of stamps, in changed colours, had been received in Brunei just before the occupation. These were not used without the overprint, though some mint examples exist un-overprinted.
In some cases, older stocks were pressed into service when more plentiful supplies of some values were exhausted. The scarcer ones are the 6c Scarlet (which replaced the commoner 6c greenish-grey) and the 8c grey (which replaced the commoner 8c red).
The other stamps in this series that are particularly difficult to find are the $5 and $25.