I am trying to restore an old oak chair that was given to me by my mother. It was my great grandfather's and is probably 120-140 yrs old. At the front of the chair seat there is a long ~3/8ths slit, like a long dado that runs about 5 inches from the front of the seat. Much of whatever had been put into the slit has disintegrated but some of it remains. It looks to be some sort of fabric or material that has long since hardened. I was thinking about chiseling out what remains and then routing a 3/8ths piece of oak to put into the open gap. Has anyone come across anything like this or any recommendations? Pictures attached.
Thank you - Brian