Another one to watch out for is fake 6LB6's, also made in Korea and branded International Servicemaster. About 2-3 years ago I bought a couple NOS for spares for my living room Zenith.
They were made in Korea had 1982 date codes on them, but were definitely not 6LB6's.
The plates were about 1/2-3/4" shorter, and the cathodes were bigger, about 3/8" wide. They nearly pegged the needle in my tester. They did work in my set, the cathode current was lower than with real 6LB6s, but they arced internally for the first 5-10 minutes after power-up, until it reached operating temperature. I've bee using one in my set for over a year now, and still going strong (and still arcing until up to temp.), but one day it's going to short completely (hopefully the Horiz. fuse will blow and not the flyback).
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