The Mystery of the Scar-Faced Beggar

A wallet dropped by a scar-faced beggar sets the three investigators on the trail of bank robbers.
In this one, the three boys do more "real" detective work than we usually see them do.
The interesting part of this book is the introduction of Hector Sebastian. Originally the series used Alfred Hitchcock as a character who introduced the write-up of the boys' adventures. After Hitchcock's death in 1980, the fictional Sebastian was introduced to replace Hitchcock. For a number of reasons, the series was later rewritten without Hitchcock's name. All of Hitchcock's appearances were replaced by Hector Sebastian. I don't know if this book was also rewritten in later editions, but at least the edition I have shows that Sebastian character was originally not intended to be written over the previous stories.