Black marks on output tubes?

I have a Scott LK72 and the 4-7591 tubes all have the same black marks on them what causes this?
Do you mean inside the glass? If so it is called a getter. It is an element that is purposely overheated by applying current to it after the tube is made. It's job is to burn up and while doing so get whatever oxygen is left inside the tube.
Yes, it's on the inside that makes sense Thanks