[old_tv_nut]

Very interesting. There are a number of things there I never heard about before.
I think it will be necessary to read the entire articles.

I never heard of hexagonal holes.

The transparency of the mask per se is not the limiting factor in black matrix (negative guardband) tubes, it's the percent coverage of the black matrix. This excerpt doesn't make a distinction, so i wonder if it is referring only to older tubes in the case of very low transparency and strong gradient.

I do recall discussion of the noticeability of the gradient in transparency from center to edge in tubes that had a strong gradient. This was of concern if a manufacturer bought tubes from different sources that had different gradients. A strong gradient is not very noticeable by itself, but two different ones displayed side by side might be.