A 1481 date for the manuscript Siena, Biblioteca Comunale degli Intronati, MS K. 1. 2 (hereafter Si), which contains almost ninety pieces of late fifteenth-century liturgical polyphony including works of Obrecht, Isaac and Mouton, has rested uneasily upon an argument from Sienese copying records. A fresh codicological analysis of this important source, including evidence of matching ‘twin’ watermark pairs in datable Tuscan archival documents, has yielded a new date and a new narrative for its compilation. The main corpus, in fact, is decades later than the fragmentary appendix containing works of Dufay and Josquin. The redating presented here has manifold implications for some of the most important composers of the late fifteenth century, and for peninsular patterns of musical transmission. A secure early date for the appendix copy of Josquin's Missa L'ami baudichon, for instance, enables a reassessment of that piece's transmission to sources as far away as Poland and Bohemia in the light of recent discoveries in Josquin's biography. My new date for Josquin's mass also confirms a case of Josquin emulation within another, anonymous mass in Si. The new dating further enables a reassessment of ModE M.1.13's authority as a source for one of Johannes Martini's masses, and the identification of a heretofore unknown local Sienese composer. Finally, the new narrative for the Siena choirbook may reveal Florence as a link in the transmission of repertory from Milan and Ferrara to Siena.