This was an ad ran in Reader's Digest in 1966 asking readers to "write in for a free booklet."
"Writing in for a free booklet" was the "email signup" of the day before the internet.
You would write a physical letter ➡ Send it via Post Office ➡ They would send you the material and collect your name/address to put on their marketing list.
There was so much more work involved in creating a "customer list" back in the day!
