[old_tv_nut]

Extremely hard to see in the posted image. I had to open it in Irfanview and increase the contrast to max to see anything.
White field impurity problems were essentially eliminated in the mid 1960s with the introduction of negative guardband tubes. I wonder if there's a phosphor saturation problem due to sharp focus varying over the image. Is there a focus adjustment you can try to see if it affects the problem?