Scottly,
If Danny's 75 petcock is still stock for that year (I think it is), then it has the nylon cone selector intsead of that 4 holed rubber thing that other petcocks have. I don't see how black bits could come from there.
What I could see of the sock in the tank looked intact. Only thing I could think of was the fuel line. But, now that I think of it, the vent lines could also feed black bits into the carb bowls. Who knows what is inside those that insects can carry in.
Danny didn't tell us if he found more black bits in the carb bowl, did he?
Cheers,