North Borneo Jap Occupation overprints and "duit pisang"

The 1942 Japanese Occupation overprint $1 and $5 are probably genuine but the overprinted war tax 1c and 2c are definitely fakes. The fake overprints are much clearer and looks more recent. They look as if they were printed with an inkjet printer. There are a lot of clever fake japanese occupation stamps out there and some of them are very difficult to authenticate. It is better not acquire any expensive items unless it is clearly certificated.
The 1939 set were overprinted with "single line chops" at Sandakan and Jesselton. They came into use in October 1942 and were withdrawn by the end of  July 1943. They were then replaced by overprinted jap stamps. The war tax stamps  were surcharged in smaller quantities and are consequently very rare especially used on cover.
"Duit Pisang" or banana money so called because of the motif on the $10 note. They were in use throughout Malaya, Singapore, Sarawak, Brunei and North Borneo. A lot were printed resulting in hyperinflation leading to a severe depreciation in its value. Counterfeiting was rampant. They became worthless following the surrender of Japan. The notes with serial numbers are more valuable as collector's item.