When Rollin makes a cast of the doctor's face, he is clearly seen putting plaster of paris over the man's mustache, but when he removes the final mask from the mold, it is clean shaven.
When Rollin and Cinnamon have to make a mask of the doctor (played by Will Geer) "on the fly," Cinnamon takes a fairly stiff piece of clear plastic and melts it in the doctor's sterilizing oven to create the mask. Such plastic, if melted and reconstituted, would have hardened into a rigid, mostly clear constituent; it never would have provided the flexible, flesh-like material of the mask depicted, which would have required some kind of latex.
The mask Rollin makes of the doctor's face has a smooth upper lip even though the original mold was made with the doctor's mustache intact.