Research of the manuscript hitherto known and labeled as the "Liber sequentiarum and sacramentarium" (“LS”), stored today in Šibenik, has shown that this chant book is the earliest missal copied in the monastery of Tegernsee for the St Thomas Basilica in medieval Pula, Istria.Indications for its provenance are drawn from its codicological, palaeographical and repertorial features. Beside the detailed comparison of its script, notation, sequence and trope repertory, this article shows up to date not analyzed repertory of the saints venerated in its sacramentary and in the list of relicts to be mentioned at the end of missal (Haec suntreliquiae). Particularly, this portions of the “LS” repertory were a clue toward detection that the “Leitheiliger” of this chant book is St Thomas Apostle, a patron of the bishopric of Parenzo/Pula, to whom also the “unicum” sequence Armonia concinnans was dedicated. St Thomas Basilica in medieval Pula is place of usage where this “commissioned book” has been meant for liturgical usage. In the broader Aquileian context this manuscript is important as a “new source” from Aquileia and its eastern province of Istria.