This harpsichord, with an inscription that has been read as “Jesses Cassus,” has been the subject of intrigued speculation since 1956, when it was mentioned and illustrated with two photos in the first edition of Donald Boalch’s Makers of the Harpsichord and Clavichord. Its apparent significance has stemmed from the interior decoration of the case, which is quite similar to that of English virginals made between circa 1570 and 1684. As an English harpsichord of the “virginalist” period, the “Jesses Cassus” would be of extreme rarity, since only three pre-1700 English harpsichords are known, made in London by Lodewijk Theewes (1579), John Hasard (1622), and Charles Haward (1683). Although many questions remain, it is clear that the instrument was heavily rebuilt, that is, made up from various antique and new parts between about 1880 and 1925. To summarize the discussion below: while the interior decoration and parts of the case may be seventeenth-century English, the soundboard, together with the wrestplank veneer, wrestplank, registers, and lower guide are most likely from a German harpsichord larger than the present case. Enough remains of the soundboard and associated components to undertake a plausible reconstruction of the original form of the instrument from which they came. The keyboard was adapted from a third instrument. With further detailed study, the authenticity of the English-style decorative elements and case parts might be established. If so, some aspects of the instrument might be reconstructable. Long in private ownership in California, the “Jesses Cassus” harpsichord is now available for acquisition by a venturesome institution or curious individual. Although quite compromised as an example of historical harpsichord building, the instrument is a fascinating study piece containing highly significant and extremely rare historical material, including substantial portions of a seventeenth-century German harpsichord, one of only a handful known.