The specs for both coins are almost identical:
5 Sen: 1.72 g, 17.78 mm, stainless steel
https://en.numista.com/catalogue/pieces23730.html
1 Sen: 1.74 g, 17.78 mm, copper clad steel
https://en.numista.com/catalogue/pieces931.html
Your first step would be to verify that your coin is not exhibiting environmental staining or that it has been plated with copper.
5 Sen: 1.72 g, 17.78 mm, stainless steel
https://en.numista.com/catalogue/pieces23730.html
1 Sen: 1.74 g, 17.78 mm, copper clad steel
https://en.numista.com/catalogue/pieces931.html
Your first step would be to verify that your coin is not exhibiting environmental staining or that it has been plated with copper.
Inordinately fascinated by bits of metal with strange markings and figures




















