Why did the color roundies have the mask?

[miniman82]

Originally Posted by Phil Nelson

Anyhow, there's nothing to prevent you from making a porthole color roundie, if you want to throw away your mask and you can get enough vertical deflection from your set.

Besides the fact that all round color tubes I've ever seen didn't have a full silk screening of color dots at the very top and bottom of the tube, I think it would look terrible. They have what looks like registration marks top and bottom.