A few years back, I was fortunate enough to have spent six days with the premier saw maker of Japan, Yataiki (R.I.P). He was versatile, could make many other tools as well as 113 saw patterns. I had to leave the workshop early, but one of the participants, John Burt, sent me as a gift, a marking knife that Yataiki made. It had the hollowed flat back, which was done with a sen. Yataiki had many sen of varying cutting blade widths.
The workshop was 30 days long and was held in Fairfield, Iowa. It was designed primarily for woodworkers who used Japanese saws, and the emphasis was on sharpening and repairing saws. I was allowed to make a small hammer under Yataiki's direction.