There are two approaches to classifying koi - the foremost reason is for entering koi into a show. For pond-grade koi, classification is often a moot points - the label only helps you communicate with other koi keepers. Since this koi isn't competetive, the label doesn't really matter.
The other reason to classify koi is as a way to understand where they came from and thus where they might be heading.
As a genetics geek, I am sometimes more interested in the biology than the koi on the day. Your koi is a sanke, despite the sumi on the head. As a sanke, sumi may continue to appear, but there won't be the kind of dramatic changes you can expect in showa. As others have said, the striping in the fins and the dorsal (top down) rather than wrapping pattern of sumi are consistent with sanke genetics rather than showa genetics (modern or kindai showa development can be a bit more complex). Looking at this sanke - the body isn't half-bad, it is isn't a specklegoi, and the white is nice and clean. I like the sumi. The pattern is certainly non-traditional for a sanke, but it is balanced. As a pond koi, it is far more attractive than many I have seen, and it seems to be well kept. Koi are a journey - this one can help you learn.