Iyo no Jo Minamoto Munetsugu worked at Isahaya in as a pupil of the first-generation , and the published sources rank him with that founder as one of the pioneers of early-modern forging. He belongs to a school remembered for the calmest of refinements, the fine konuka- and quiet that Nabeshima patronage made the standard, and within it he is the deliberate exception. Where the main line tempered a straight that turns into a small round , Munetsugu forged a vigorous irregular temper in an emphasized - manner, and the commentaries return again and again to that contrast as the thing that fixes a blade in his hand. The early name Muneyasu, which the old reference works treated as that of a disciple of the first Iyo no Jo, is held in the prevailing view to be the first signature of the second generation, named by the lord Nabeshima Katsushige in the ninth month of 'ei 9.
His characteristic hand is a built on and , conspicuous for the pointed mixed within it. The published sources name the as a hallmark of his varied temper, and it is the - point, not the soft clove-head of , that separates him from the orthodox . Over the temper run vigorous and frequent , the deep and the thick, with , and breaking out above the ; the upper half characteristically turns more floridly than the lower. The is the surest tell of all. The mainline point runs straight and rounds in , but Munetsugu's almost always enters and sweeps back in , at times flame-like, so that the can say of one of his that the irregular makes his work unmistakable among other blades.
The carries the school's fineness without its restraint. He forges a fine , often a closely packed that takes on a flowing tendency, the surface covered with thick and threaded with ; on the broader pieces the grain stands a little, and a touch of larger appears. The steel reads as the refined material the published sources describe as excellent in quality, but it is made to serve a - temper rather than a quiet , and the and together are read as richly covered in and assertive in spirit. Even the tang is part of the portrait: he thins the flesh on the edge side to give a fish-belly tendency, and against the habit of signing the he cuts a bold long signature on the , in fine chisel yet large and expansive, a hand the commentaries call distinctive on sight.
The corpus divides cleanly by name across two generations who share a single manner. The first generation signs Munetsugu, most fully as Iyo no Jo Minamoto Munetsugu of , and the published record makes him the maker of the line's finest pieces. The second generation signs the early name Muneyasu before assuming the Iyo no Jo Munetsugu title, and his blades, few in number, are valued as much for what they settle as for their quality. The Muneyasu commentaries quote the Bengi, which says only 「伊予掝(初代)門人ならむか」, perhaps a disciple of the first Iyo no Jo, and the Ichiran and Kokon Kajibiko, which likewise treat him as that disciple, then set the modern reading against them: that Muneyasu is the second generation's first signature, corroborated by a surviving naming warrant from Nabeshima Katsushige. The reverse file marks are the other generational divider the texts name. What does not change between the two is the workmanship, which the published sources treat as a single shared range, so that a Muneyasu and a Munetsugu read alike to the eye.
That shared range has a stated model. The published sources say the first generation customarily took old works as his pattern, and they describe the Muneyasu pieces as aiming at in the - style, the fine giving the a soft, luminous brightness and an unforced, archaic taste. The model places Munetsugu against his own school by his own grounded traits rather than by anyone else's. His is the bright, -laden - with conspicuous and an irregular swept , set inside the very school whose name stands for refined , and the resulting individuality is exactly what the commentaries mean when they call him the most distinctive of the smiths and his work unmistakable among them. The hallmark and the carry across both generations, which is why the second generation's Muneyasu blades are read as confirmation of the first generation's hand rather than as a departure from it.
He is a Jo-jo smith with a thin but high record: one in the and ten in the , against no National Treasure and no Important Cultural Property, so that the designated blades on record number around a dozen and reach the market only rarely. The summit of that record, and of his life's work, is the forged through the silver that the third shogun Tokugawa Iemitsu bestowed within Kyoto on his 'ei 11 visit to the capital; Nawa Sotan, of the lineage of Nawa Doen, a senior retainer of Tokugawa Yorinobu of , applied part of that bestowed silver to commission it, and the order inscription with its date of 1634 makes the piece, in the words of the published commentary, his finest achievement across his lifetime and a document of exceptional value. Beyond that named sword, provenance is sparse and best left as recorded rather than embellished; no current museum or shrine holding is on record for his blades. For a collector, a Munetsugu or a Muneyasu is among the more individual things early can offer, a - exception inside a school, encountered from time to time and with patience among the blades, a landmark of the line when one appears.