As suggested by discrepancies in its position within the various Mozart catalogs, the Symphony B flat major, alternately known as No. 55, K. 45b, and K. Anh. 214, is of unknown date and provenance. No scores or parts for the piece were available to Köchel when he made his initial inventory and only a melodic incipit in a thematic index indicated the piece's existence. An inscription on a set of parts found years later bears the date 1769, but stylistic traits suggest a somewhat earlier placement. Some scholars even call into question W.A. Mozart's authorship of the work—an issue impinging on a handful of these very early symphonies composed at the time when young Wolfgang's compositional skills and notoriety were just beginning to clearly eclipse those of his father Leopold. Among the traits used to estimate the work's date of composition is its four-movement structure. This characteristic appears in the early symphonies K. 43, K. 45, and K. 48, as well, all of which were composed in 1767-1768 during an extended stay in Vienna; the shift from three- to four-movement symphonies was a deliberate appeal on Mozart's part to the formal sensibilities of Viennese audience. Likewise, this same period saw Mozart undertaking a thorough mastery of first-movement sonata-allegro style. While the K. 48 symphony is the first to feature a full-blown sonata-allegro form in its first movement, the Symphony B flat major (K. 45b), does feature something approaching it; in fact, the opening Allegro is structured as a kind of musical palindrome, with the recapitulation revisiting the themes from the first half of the work in reverse order. Like the first movement of the K. 48 symphony, this movement is also somewhat unusually cast in 3/4 meter, and the opening themes of both symphonies feature rather plain melodic contours enlivened by sudden shifts between forte and piano. (Another important feature of the opening movement connects it to a much later work: the "1-2-4-3" melodic gesture in the bass that appears later and more famously in the Finale to the "Jupiter" Symphony.) The second movement, a delicate duple-meter Andante, is brief and poignant, again employing sudden dynamic shifts to further shape its melodic contours. A stately Minuet and Trio third movement leads to the finale, a quick 2/4 movement in binary sonata form marked by decisive articulation and reverse-dotted "Scotch" rhythms.

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