Steve discovered this clever advertising ploy from Harpers of the 20s and 30s. While he was trolling the Hathi Trust site for an old cookbook, he found an example of a Harpers Sealed Mystery. These books had a seal enclosing a portion of the last pages (said portion determined by the author). Harper promised to refund the purchase price of any HSM that was returned with the seal intact. I'll bet this helped loosen the purse strings of many a mystery fan. Very few books were actually returned.
I checked the catalog and found a few titles from the series in Widener. These 2 still have the front part of the seal. (Larger versions of these pictures available here.)