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Metal cones were popular here too in the late 40's early 50's.
There's at least one 8" type, several 16" the 21AXP4 color tube and of course the 30BP4 used in the Dumont Royal Sovereign.
It would seem that size matters in how well they hold vacuum, all the 16" tubes I've had including a 50 year old, NOS in the box 16AP4 were just fine, many of the 21AXP4's seem to have gone to air as well as a lot of the 30" tubes.
There were some rectangular tubes with metal cones too, I don't know their numbers off hand.
The plus side of the metal tubes is they are very light, also the face plates are precisely made so there's no distortion.
Someone here had a 21AXP4 successfully rebuilt, it was under vacuum to start with though.
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