From the mid 1960s to late 1990s, David Klarner actively corresponded with other mathematicians about topics in recreational mathematics.

Several times, Klarner contributed material to Martin Gardner for use in his books or his celebrated "Mathematical Games" column in Scientific American magazine.

Here's a brief look at some of this correspondence.

Jan 1987: Brick Packing Correspondence

In early January 1987, Klarner sent a five page letter to Gardner about brick packing problems [page 1] [page 2] [page 3] [page 4] [page 5].

Here's Gardner's response, dated 16 Jan 1987.