Daz, I have to dis-agree, sorry.
I have 2 spares here from Stern, which I took photos of for a X Men thread, and they are painted moulded rubber. Stern also confirms same on invoice which states moulder rubber figurine.
Look inside them, and squeeze them when its warm, they are rubber. Hard on outside, soft on inside.

http://aussiepinball.com/index.php?topic=10493.0

I would bet snake head will be rubber as well, otherwise plastic will crack unless it has some flex in it.
I intend on buying a spare Sparky n snake head for a Metallica Pro, so will see if its the same as X Men toys.