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I have the Riders and Sams for the 505/T-54. Both show the 6SH7 version.
If I'm remembering correctly, and these knobs are made of the same stuff, the material is breaking down internally, not being attacked by some evil mildew from the outside. That doesn't necessarily mean that sealing would be useless. Perhaps if you cleaned the knob and dried it really well, sealing it from moisture and oxygen would keep things stable for a long time.
People try all kinds of tricks to keep Predicta screen covers from breaking down. I polished mine with Novus #2 about 10 years ago, and it still looks the same. Not being a chemist, I have no idea whether these knobs are made of the same material.
My 505 project turned out fairly well, so I'm eager to redo the T-54, but there's a color set ahead of it in the queue.
Phil
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