I must admit: not really my field of numismatic expertise. Malaysia has a long and interesting history of using tin as a currency metal, covering hundreds of years.
Refer to Google Images: "palembang coins", to see some good pictures of genuine coins.
My first thought was: counterfeit. I have a few genuine tin pitis coins of this general fabric in my collection, to compare with the pictures seen here. It is easy for a backyard workshop to produce counterfeits, because tin is very easily handled at low casting temperatures. It seems to me that the coins pictured may have come from a wooden mold. The obvious casting seam around the edge is what I would expect to find with a poor quality counterfeit.