In the April 10, 1997 Sotheby’s catalog the comments preceding the Phyllis Sharpe Memorial collection of early tsuba include the following:
“N.B. Masayuki Sasano always insisted that the ‘grading’ he gave in his hakogaki reflected his personal reaction to the tsuba, and was not an attempt to provide a universal quality rating system. His grades, as mentioned in the text which follows, are: Ka: Beautiful; Shu: Superb; Kei: Masterpiece.
These grades appear toward the bottom of his hakogaki to the right of his two seals.
His highest ranking, masterpiece looks like this:
I don’t know where the reading “kei” in the catalog comes from, the first character is normally read ketsu, or when combined with saku as above kessaku. It can also be translated as greatness or excellence (and can be used sarcastically to refer to “an amusing blunder”).
His middle ranking, superb:
And the “bottom” grade ka:
Although the dictionary definitions of kasaku include “good work” and “honorable mention” which sound somewhat less enthusiastic. I’ve never seen a poor or uninteresting tsuba with a Sasano hakogaki, so I think beautiful is fair enough.
I agree with the catalog notes that this is a personal and subjective system. From what I’ve seen I’d guess that Sasano-sensei was thinking about the tsuba in comparison to others of their type or at least definitely not imposing a hierarchical view where some groups “outrank” others. The kessaku example at top is for a small nidai Akasaka guard, the shusaku is for a very small and simple yamagane ko-kinko tsuba with kiku and other stamps and the kasaku is from the Yagyu tsuba in the previous post.