The latex part of the Spiderdemon is actually a recast of the original model, and said recast was also made by Gregor Punchatz himself. What happened was that the brain/body part of the Spider Mastermind was a bit beat up after the photoscanning process that every clay/latex model went up for thedevelopment of DOOM. So the boys at id software contacted Greg to recast the body part of the model to attach it to the original mechanical chassis. Therefore the Spider Mastermind sitting at id’s offices is half original.

source; http://doomwiki.org/wiki/Development_of_Doom