I strongly suspect you're on the right track, but if "the topic" is SF then I still think that "having seen one" in the 1950s still means an issue of SF5Y, as it would be rather peculiar to boast of having seen some SF back then. But then, this is a peculiar letter.

Also: the review's "starter kit" and "first time readers" could be misread as suggesting that you sell SF books, aimed at a general market.