During the early part of WWII, the US had fears that Japan would overrun Hawaii. If this occurred, large sums of currency could be captured and used to fund their war effort. So, the US decided to issue the same $1 Silver Certificates, $5, $10 and $20 Federal Reserve Notes as used on the mainland, but with a brown seal and serial numbers and overprinted with the word "HAWAII" twice on the front and in large block letters on the back. Because these notes were distinctive, it would make it easy for the US to demonitize the notes if large amounts fell to the enemy. Later in the war, these notes were used in the US held Pacific Islands for the same reasons