Is the "current number of titles" count that
sometimes appears on the back panel reliable?

In a word, no. You'll often see phrases such as "Your choice of 212 great titles" or "265 great titles to choose from" or "Over 300 titles to choose from the world's great literature" on the dust jacket back panel. But an inspection of the more reliable list that appears on the dust jacket inverse shows a different number

The list of titles that often appears on the inverse of dust jackets and at the back of book blocks is a much more accurate way of determining dates. See Barry Neavill's Modern Library Dating Key article.

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