One of the joys of compiling a thematic catalogue is that a number of small mysteries crop up as one evaluates biographical and source material for the composer concerned. The journey often requires delving into the private life and works in intimate detail, and brings to light important information that in turn raises questions about the various influences or connections affecting the composer or style. But when this information is lacunar or based primarily upon circumstantial evidence, it provides only plausible suppositions that, while not entirely provable as incontrovertible fact, nonetheless dare the scholar to prove otherwise. Such is the circumstantial case for the meeting of two of the more remarkable composers of the second half of the eighteenth century, Wolfgang Amadeus Mozart and Joseph Martin Kraus, who share nearly identical life spans and dates.