Niewmismatic Error Coins

Undated Convex Shape Terengganu Tin Pitis/Keping
Rarity: Unique/Unlisted/SS33                          

Terengganu Pitis/Keping Estimated to circulate in the state of Terengganu around late 18th century until early 19th century.
This is an unlisted Pitis/ Keping (similar to SS33) casted in convex shape. I was lucky to inherit it from a very senior collector from down South during my recent trip to Singapore.
I welcome your ideas on how it was formed in convex shape. Thank you in advance.
Obverse in Arabic : Kali Malik Al Adil ( Translation: The Reign of the Just Ruler) In Convex shape.

Reverse: Blank (Concave shape)

Weight: 1.82gm.

Diameter: 20.00mm
Edge: Plain

Composition: Tin

Source: The Encyclopaedia of The Coins of Malaysia Singapore And Brunei (Saran Singh)