Just seen this topic.
That is definitely a silicone piece, the seams are quite visible, probably a test piece to get the head armature right. I imagine it was done using a mould with an internal plug the same shape as the armature, then outer sections for the actual features.
There's a sort of chicken-and-egg which-comes-first question. If you sculpt the head in clay first, you have freedom to make it any shape you like. Then you need to line the mould made from your sculpt with rolled-out clay and cast some plaster inside to get the shape of the armature skull. This is then created in whatever material you choose, inside a mould struck from the plaster shape. The plaster shape can then be used as the internal plug in the original mould of the skin, to result in a silicone skin that fits neatly over the armature.
You could create the armature first, but would need to imagine the overlying skin and facial features. Then you could make a plaster plug from this, onto which you sculpt the facial features. Or just wrap the armature in some cling film and build up with clay. But this way round would restrict your choices and might end up not quite how you intended.
The other way is to dispense with the plug and just cast the silicone using an armature that is intended to remain embedded, like StopmoNick shows in his video. But Ron Cole had some very complex controls in his heads, so presumably needed to slip the skin on and off.