The fixed point in Masanobu's record is a signed Bishu Masanobu and dated Meitoku 5, that is 1394, the long signature cut in fine chisel strokes on the and the date carried on the reverse. He was a swordsmith of the Ko- school in Bingo Province, working at the close of the period, and a second blade carries the still earlier date of Eiwa 2 (1376), the two dated pieces fixing his activity within the final decades of that age. The published commentary counts him, together with Masaie and Masahiro, among the representative hands of Ko-, and names him with Masamune of the same group as one of the smiths in whom 「正宗・正信等はその最後を飾る刀工であろう」, those who adorned the finale of the line. The group arose at the end of the period and flourished down to the close of the period, and the work of its late--through- phase is collectively called Ko-; the strong Yamato temperament of that work is explained by exchange with the Yamato heartland through the many estates of its great temples and shrines, Toji and Rengeo-in among them, that lay within Bingo.
His hand is the school's hand, and it is a quiet one. Over an mixed with and flowing that stands rather than lies flat and runs toward along the edge, he tempers a low, gentle straight temper, a or that here and there carries a shallow and mixes in small and slight rather than working itself up into a true . Along the run , and futae-, the doubled temper line, with small entering and fine and threading through; gathers on the line, and the is tight and inclined to sink rather than to glow, though on the best blades it brightens and clears. The published sources are candid that the temper is subdued in feeling, yet they hold that 「直刃の出来も地味ながら同派の持前をよく示して」, that the , plain as it is, displays the inherent traits of the school well, and that the workmanship is sound throughout. He is a smith read by the restraint and evenness of a straight edge, the small a recurring punctuation along an otherwise still line.
The is where the school marks itself most plainly. The stands, the grain visibly raised, and the steel takes on a bluish-black tone over which a whitish rises; fine adheres, and -like dark lines enter the standing grain, and patchy texture gathers in places. This pale, standing , with the flowing turning to and the gentle, rounded that runs straight with to a or an return, is what the judges have in mind when they read a Yamato temperament throughout work. They put the school manner plainly: compared with Yamato proper the of both and is generally weaker, the stands out and the grain rises, the is conspicuous, the inclines to tighten and the turns back in a calm, rounded form. Masanobu's blades hold to that base almost without exception, the bluish-black steel deeply tasteful and the tight- bright and clear on his finest piece.
His surviving work falls into two registers, and the rarer of them is the one signed. The signed pieces are or only lightly shortened cut with a long Bishu Masanobu signature and an Eiwa or Meitoku date, and they are exceedingly few; signed Masanobu, and especially signed-and-dated examples, were thought from old times to have almost no precedent, so much so that among the -period sword reference books one records 「この太刀のみを載せている」, this alone. One such piece is the unusual construction, which the published sources grant is encountered from time to time in a so strongly touched by Yamato. The larger register is the or blade, with the tending high, on which the folded-back three-character signature or the bare attribution carries his name; it is on these that his work is most often met, and the standing whitish , the and the futae- that he and his school share are what hold them for him. The sword-signature compendia, working from the dated pieces, place his activity in the Eiwa and Meitoku years, securing him within the passage.
What sets him apart is best stated through his own traits rather than against another school. The published commentary, citing the Shinkan Hidensho that appearance 「面ぶり備中太刀に似たり」, resembles , grants that some of his blades read at first glance like the neighboring work; yet it keeps them for , finding that 「鎬高の造り込みや地に現われた白け映り、また刃中の二重刃・喰違刃等に一派の特色を見出し得る」, that in the high- construction, the risen in the , and the futae- and within the the characteristics of the line can be discerned. The futae- is itself a school habit rather than a personal flourish, for the records note that Masanobu has other extant examples showing it and that his fellow smiths Masaie and Masahiro work in the same manner. The same texts that praise his blades are honest about his individuality: in his work, they say, 「地刃に同工の特色と云うよりも同派の特色が濃く」, the characteristics of the school are stronger in both and than any distinctly personal style, so that he is recognized first as a Ko- hand and only then as Masanobu.
Masanobu is a connoisseur's name rather than a collector's quarry, and the record is honest about its scale. The Toko Taikan values him at a middling figure, and his blades on the official record number four, every one of them at the Important Sword tier; there are no National Treasures, no Important Cultural Properties and no among them. What survives carries provenance of the first order: the Meitoku 5 descended in the Maeda house of the million- domain, its mid- black lacquered mounting still attached, and the was transmitted in the Ii house, lords of the Hikone domain, the published sources calling it 「現存稀な古三原正信の在銘作として資料的にも貴重である」, a rare extant signed work by Ko- Masanobu, precious as documentary material. Most designated blades of this rank are held rather than traded, and a signed and dated Masanobu, of which only a handful survive, reaches the market very rarely indeed; the and pieces are the more findable face of his work, and one appears from time to time, offering the student of the schools a sound, characteristic example of a Bingo Yamato-influenced hand, the kind of grounded blade on which a careful is built.