Could it be asbestos behind refactory in heatilator fireplace insert?

The firplace insert in my 100 year old home looks to be 30 - 40 years old and the steel refactory pannel is buckeling and revealing a white fabric mat. Could it be asbestos? what else could it be? The insert is a Heatilator Mark 123 model #3138. The Steel refactory pannel is bolted to the back inside wall with the bolt heads exposed to flames.