Need help NON mopar 1967 Pontiac Lemans 326

That might be a weird mark,but IMO,that's a cracked insulator. It might idle on that, but as soon as it has to work, it will cut out. I've seen that on a lot of cars; just replace it.
If there's one there's often more.
That's not a defect, there is something wrong in that hole. Take note of which cylinder it came from and see if it happens again. I've seen pre-ignition start that way. The next thing that can happen is the porcelain will shatter, and that stuff can wreck your engine. I suggest to back off the timing until you get to the why of it.