I'm not sure that I have ever read of a period when they were not "good". Whenever I see one on craigslist, I email the owner and ask for the serial number if it looks interesting. I haven't found the perfect one yet. I fix and rejuvenate older/vintage machines, but because I don't particularly need another machine, I haven't purchased one yet. I may never, but I do know that most I have seen out there at vintage and made between 1935-1950.
You should join the vintage area and ask the people there. They are so helpful.... Go to the home page..scroll down to the vintage area and subscribe...