I'm confident the tube is too gassy to use in its present state. The gassy 15GP22 I tested in a CT-100 here in the Detroit area also has that tell tale blue glow between the gun elements. Sometimes the voltage of the tester itself is enough to strike the gas and get discharge like that.
Reread my earlier post: blue glow on the inner surface of glass = harmless.
Blue glow BETWEEN elements = bad.
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