My guess is the glass was inspected for flaws before phosphor application if one near the edge was found mark it to be bottom or top in the masked off area like that, then apply the phosphor and off to the races of the rest of the build.
I take it you used the heat gun method (which is the least safe)...I'm thinking of trying that today on a couple RCA style cats. Been using the clothespin and sun method on the RCA style and I can't seem to get the finesse right to not crack the glass.
I like the secondary cat a lot better.
All you probably need to get that one off the tube is some food and followed by windex and paper towel for the fuzzy residue they leave.