I thought I heard Dale of cycles d'oro say something to that effect i.e. he's seen very little internal rust in frames that outwardly appeared good/very good. But he said alot that day, so I could be misquoting him.
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1989 Schwinn Paramount OS
1980 Mclean/Silk Hope Sport Touring
1983 Bianchi pista
1976 Fuji Feather track
1979 raleigh track
"I've consulted my sources and I'm pretty sure your derailleur does not exist"