Krause lists these coins as bronze. However, "The Coins of Malaysia" (1969) by Kevin Kavanagh and "The Encyclopaedia of the Coins of Malaysia, Singapore and Brunei, 1400-1967" (1986) by Singh Saran list them as made of copper.
References to additional information:
[Book] Atkins, James. 1889. The Coins and Tokens of the Possessions and Colonies of the British Empire. p 231.
[Book] Kavanagh, Kevin F. 1969. The Coins of Malaysia, pp 61-64.
[Book] Remick, Jerome. 1971. The Guide Book and Catalogue of British Commonwealth Coins, pp 82-85.
[Book] Saran, Singh. The Encyclopaedia of the Coins of Malaysia, Singapore and Brunei, 1400-1967. Kuala Lumpur, Malaysia: Malaysia Numismatic Society, 1986. pp 373-411.