We recently discussed some marbles which looked like this. This one was put into boiling water which softened it so that pieces could be pinched off. The pieces were sent in for laboratory tests.
The conclusion about this one, if I understand correctly, is that it is not a polymer material. Rather, it is a cellulose material with wood and some rosin. That would mean it is not gutta-percha since gutta-percha is a latex, which is a polymer emulsion. (Again if I understand correctly.)
The question now is, do you have any ideas what the name of the material might have been? Or any names at all for any old materials which you even suspect might be related. For example, the name Elastolin has been suggested. Any others?
I'm not clear on whether this mib has any clay in it. Need to double check. Maybe Snyd will weigh in.
A few wiki links for perusal:
http://en.wikipedia.org/wiki/Gutta-percha
http://en.wikipedia.org/wiki/Latex
http://en.wikipedia.org/wiki/Rosin
http://en.wikipedia.org/wiki/Elastolin