I have been working in an old building, I'm guessing maybe 50 or 60 years old, but that is just a guess. I came across a wire type that I'm not familiar with. The conductor itself appears to be copper with a tin or lead coating, but is clearly copper inside when cut. The insulation is a layer of rubber(?) , then a fabric layer which covered by what feels like tar(?), I'm not sure exactly what it is. Anyone know what this is? I need to determine it's ampacity. There are no markings and looks like #6