from UR Research ( University of Rochester) web page : "An unpublished work. --- An account of this composition, attributed to Pergolesi by Humphrey Claydon, is published in Music and letters, Oct., 1938."
Grove Music has 2 sinfonias in B♭ in its work list for Pergolesi; one is "doubtful" and the other is "spurious . . . by J.G. Graun".