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The problem is the barely visible, cloudy area just to the left of the silver portion of the lower gun. That is where the very silver tube getter should be.
The getter was a compound applied to the neck wall of a CRT (and most small tubes) to absorb the last molecules of gas in the tube when it was built.
The problem with 15GP22 tubes is that the front of the tube containing the phosphor screen and mask was attached to the bell portion of the tube with a sealant. Over time most of the tubes have developed a leak at that point which allows air in and eventually overwhelms the getter until it starts to go from silver to chalky to almost gone. Much like the white powder in small tubes that appears when it cracks or you drop it.
It is the quickest indicator of the condition of a tube without testing.
Dave A
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