I received an awful lot of answers to the question I posed about the membership of the Legion of Super-Heroes at various times and what that might tell us about the origin date of that DC Comics form letter I shared with you. So I now have a mailbox full of messages telling me, for the most part, all the same things.
It comes down to the inclusion of Ferro Lad in the then-current Legion roster. He joined in Adventure Comics #346, published in May of 1966 and died in Adventure Comics #353, published in December of 1966. But of course, those comics had about a four-month lead time in going to the printer and anyone working on the comic in the DC offices would have known months before that of what was going to happen in the continuity.
So I'm going to say that the mailer was probably printed around January of 1966…which tracks well with the mention that a Superman cartoon show would be debuting later that year. That's about the time the network would have firmed-up its Fall schedule and production on the show commenced.
My pal Tom Galloway noted a mistake in the mailer: "It lists only five types of Kryptonite, and doesn't include Jewel Kryptonite, which was introduced in Action #310, cover dated March 1964." And I think whoever wrote the little note to me was wrong when he or she — and I'm pretty sure it was a "he," Nelson Bridwell — wrote that the inking on their comics was usually done before the lettering. I think he meant to write that the other way around.
Meanwhile, Steve Thompson wrote to inform me that he had a different version of that mailer and he featured it on his blog. This one was done after the one I posted but not too long…but I'm not going to get into how long. Thanks to all who sent in info!