Thanks Bob and John for posting the construction details of the 15gp22. Wow! No wonder many of these crts leak. I suspect that percentage wise, more prototype metal cone 15" crts have survived!
I suspect that patching over the outside of the seal areas with frit (or glass) would not be a permanent solution. as flexing of the seal area is likely to fracture these brittle materials. Also, I suspect that pockets of air that could be trapped between the frit and the metal flange could be a source of a "virtual leak" that could over time cause the tube to become gassy even though there may be no detectable path to the outside...
Tough problem !
jr