The coin seems like to be an sea salvaged coin. Most of the legends are present in the coin under a black layer. I am not a specialist so cannot confirm that.
Details from Numista website
Numista Rarity index: 70
| Issuer | Madras Presidency (British India)  |
|---|---|
| Type | Standard circulation coin |
| Year | 1797 |
| Value | 1⁄96 Rupee |
| Currency | Rupee (1691-1835) |
| Composition | Copper |
| Weight | 6.67 g |
| Diameter | 24.5 mm |
| Shape | Round |
| Orientation | Coin alignment ↑↓ |