In 1943, The Imperial Japanese Government returned 4 Northern Malay states (Kedah, Perlis, Kelantan, and Terengganu ) to the Axis-aligned Thai government as a reward for entering into a military alliance with the Japanese.
The reason behind The Imperial Japanese Government say " returned" because the 4 Northern Malay states (Kedah, Perlis, Kelantan, and Terengganu ) used to be tributary states of the Siam Kingdom which would later become Thailand.
The Rattanakosin Kingdom sign the Anglo-Siamese Treaty of 1909 by giving the 4 Northern Malay states to the British in order to safeguard the Siamese independence and to gain the British Support.
This is a picture of Plaek Phibunsongkhram at Hyde Park, New York, 1955
Some interesting fact, Plaek Phibunsongkhram serve in 2 types of government throughout his life, a Fascist Thailand government during World war 2 then later a democratic Thailand government after world war 2. Not easy to last that long in politics especially through a world war.
Then-Prime Minister of Thailand, Minister Phibunsongkhram issued a declaration that the citizen of the annexed states will retain and be granted equal treatment henceforth. The Thai government then went on to issue new currency to the four states. To their dismay, the new currency was abruptly rejected by the Imperial Japanese since the Japanese were controlling power and still retained a significant degree of control over the Thai government.
The Thai government-issued currency of one dollar banknote and three denominations Minted Tin coins; 1 cent, 5 cents, 10 cents, were never released and circulated as the Imperial Japanese insisted the continued usage of their Imperial Invasion Currency.
After the war, the Japanese occupation was defeated, and The Japanese Imperial control was no longer exercised in Thailand. This ultimately leads to the Japanese invasion currency no longer being used, and as a result, annulled, Thailand was finally short on currency notes.
The shortage of currency notes in Thailand lead the Thai government to re-surface their originally printed notes of One Dollar, to be used as "Emergency" notes at that time, by overprinting "Fifty Baht" over the top. These emergency notes were only issued in Thailand.
(All information is from Malaysia Brunei & Singapore Banknote & Coins 8th Edition by KN Boon)