I BELIEVE that is Dynaglas or other short strand Fiberglas reinforced body putty. Not Kitty Hair the long strand stuff for sure. I've made bondo bucks from it before but not a full surface hammerform.
Here is a link for similar stuff:
https://www.amazon.com/Fibreglass-Ev.../dp/B0000AYHHN
Here is a link to a Bondo Buck made from this stuff. It is not a full hard surface but the principal is the same. It's touch stuff and would hold up well for a hammerform. In addition, once made, it could be further reinforced to make it even tougher.
http://allmetalshaping.com/showthrea...ighlight=Bondo
For me, in this particular buck, the hardest part was making the metal structure in such a way as it would come apart without being locked in place.