It does look like a prototype/concept of the Chatterer.......did studios do a few different variations on masks and then decide which to use? On the very last pic in the post above, the missing "eye" has a knot type shape to it just like the screen used chatterer, this also has the same type of mouth setup.....its almost like this was the basic design for him and it was then refined down to the final mask. Aside from the mouth needing to be smaller, the nose area smoothed over and the other eye removed, its pretty close!
At the four corners of the mouth there are indents where "hooks" could go in to pull the skin back, same on the back shoulder sections, where the skin has been peeled back to expose the muscle underneath, again with indents for hooks to go in?
I would have thought if it were fan made by someone with this quality of sculpting skills, then they would have made it accurate and not added an eye, nose and made the mouth so much bigger.......also, the guy I got it off did genuinely appear to have it in his loft for all those years (it was well buried in there, took ages to find it) and really didn't have a clue what it was, so I think it is probably from the late 80's like he claimed.
I'm new to all this, only just started collecting, so I don't really know how studios go about designing the masks that they use....but if they did make a few different versions of each type, then maybe this is one version of the Chatterer that was rejected, hence it ended up in the skip behind the studios, like the guy I bought it off claimed it came from?