Foreign Stamps in Brunei

The above is a Sarawak stamp complete with a portrait of Charles Vyner Brooke. Look closely and you will see the word Brunei handstamped onto the stamp and dated sometime 2nd March 1946. For those of you who are both history challenged and postal knowledge challenged would wonder when we used Sarawak stamps in our country? And if that surprised you, look at the stamp below.This is a North Borneo