Some amazing information nembiak...
Ringgit Kepala - the KWI rijksdaalders did not circulate much. Are you sure the name wasn't intended for KWIII? He has a full beard that would be unusual enough in South-East Asia. Yet, many big silver coins would have a portrait: Straits dollars, HK dollar, Spanish pesos, US trade dollar. Only the British trade dollar is portrait-less
Ringgit Tengkorak - this one is easy. Even when only slightly worn, the king looks bald, like a skull.
Ringgit Meriam - neat parallel to the Arabic nickname for this coin: abu midfa (father of the cannon). I could even imagine that the nickname travelled with Persian merchants, sailors and the spreaders of islam.
Ringgit Garuda - probably the pesos of independent Mexico, with an eagle on the reverse. Also evidence that the US trade dollar, which also had an eagle on the reverse, did not take hold in circulation.
Peter