Potentially asbestos?
This may get you on the right track:
"The Forty-Eight Insulations Company was founded in 1923 in East Aurora, Illinois. Forty-Eight sought to produce high-quality insulation." "Forty-Eight used asbestos until 1982, long after it was known that the mineral was toxic. The Forty-Eight Insulation Qualified Settlement Trust was created to compensate the victims of Forty-Eight. Two separate amounts were placed into the trust fund. The first was $39 million to resolve existing claims. The second was $15 million for future claims."

You have a local testing lab that can sample the material and give you an answer.  In my area the test runs about $50.