Rivkin

Can't say anything without better pictures. It does look like very Yamato-ish Yamato Shizu, the key question is jigane any good.

Clear picture showing sugata would help a lot, but otherwise - I see nothing obviously wrong with the attribution. Weak hamon, ayasuhi hada - not too many other options are here. Nakago is ok. Not burnishing above shinogi is a choice, and often a fine one. Theoretically it is possible that with detailed photographs the signature comes out as very weird and its Muromachi (Tembun) Gassan with a gimei Gassan signature. Such things do happen - mumei from birth and then someone quickly "corrects" the omission and we have a genuine blade with a horrible writing. Another possibility it has a hagire, running off hamon or other issue the seller does not want to show. But again it is most likely Muromachi Gassan.

I do not think there are any two utsuri which are exactly alike, especially when done not by the same smith. There are Muromachi hitatsura works which have very dark, utsuri-like patches; distinctive property is that jigane in these areas is visibly subdued. This one however to me this looks plus/minus typical Kiyomitsu work. The second image for shirake utsuri here: http://www.ksky.ne.jp/~sumie99/utsuri.html is also sue Bizen (Kiyomitsu). It has quite a bit more dispersed white utsuri on top of antai, but its still a visible, recongizable "band" of utsuri. I think one of the differences is that here the polish is more modern, more aggressive, with a specifically more accented contrast. Were this polished by Honami Nishu the white band would be more faint and would have more gradual boundaries. By default Kiyomitsu is associated with shirake utsuri or bo utsuri, so I've seen the lineage producing clear midare utsuri as well. Ofcoarse since it involves terminology we can name this one something else, but to me the differences from other Kiyomitsu pieces are not exceptional.

Yes. It might even approach nie utsuri if the crystals are bright and their location is well defined.

Shirake utsuri

The best shinshinto is typically imitation of Soshu - can be Naotane, Naokatsu, Kiyomaro. I personally would consider Naokatsu to be most affordable of the three, with quality often superior to Naotane. Kiyomaro's best works are shinshinto's very best, but he also made some rather coarse stuff where the goal was basically make it as koto as possible and there are a few simply failing to reach the top grade.

I am not a believer in attribution being strictly reflective of quality, in the very least I have not seen people with such philosophy winning kantei competitions. There are blades which are borderline Shizu/Yamato Shizu, but the majority of contested Yamato Shizu attributions go mainline Yamato as an alternative. Yamato Shizu attribution presumes strong masame below shinogi and relatively calm notare-gunome hamon with kinsuji, but few to no distinctive togari. Accordingly if gunome is quite periodic and the same size an alternative attribution is Shikkakke. If there is something strange (say, strong midare utsuri) an alternative attribution is Senjuin since Senjuin accumulates all unusual options, but also because there are verifiable Senjuin smiths who worked in such diverse styles. The best Kaneuji would be much more itame dominated below shinogi, with nice bright jigane and very powerful and stylistically varied nie activity. The best Yamato Shizu would have, for example, very uniform masame mixed with thick chikei and uniform, consistent nie deki hamon. So Kaneuji and Yamato Shizu's very best can be going into two rather different directions. Yamato Shizu Kaneuji is a rare attribution which can presume this is an early school's (i.e. late Kamakura) blade which supposedly was made by Kaneuji before his switch to Soshu style. There is one challenge is that oshigata signed [Yamato Shizu] Kaneuji show later (1360s) examples,so the notion Kaneuji was first Yamato style and then Soshu is reasonable in theory but might not be very useful in practice. A lot of top class Yamato Shizu works are from 1360s - and so are the ones attributed as "Shizu Kaneuji". Another example of sort of dealer speak when one tries to push the idea of "the earliest, the best, the famous, and the Masamune student" while in fact it is simply a very good blade and the rest is a conjecture upon conjecture.

I am sort of in the beginning phase of digging through my Kaga information for a writeup, so things will change, but: I would not trust the genealogies 100%. Kashu Sanekage is very seldom reattributed to any Kaga smiths. Despite considered the founder, his typical secondary attribution is ko Uda. Tomoshige is more or less consistent in terms of work style, it seems to be either Yamato shikkake or maybe Kinju related, the best items tend to be from Oei. Kiyomitsu and some other mitsu are mostly Bizen imitators. it might have something with the name they chose. Yukimitsu is basically the most Soshu of them all. Considering the great number of Yamato, Aoe and other smiths migrating at the time to Echizen area, there is a chance Tomoshige and Kiyomitsu might have other relationship than father/son, for example.

The greatest problem of mino all its best smiths were going full soshu, rai or even bizon of yamato

Nakago was cleaned recently, i.e. the rust inside characters is actually quite old. I am 80% for the signature being authentic. Probably 1670-1730, might be a lesser known generation in 1700s, it is often the case as sword market collapsed completely 1705-1710. Not a typical style for the school, but one of Edo period's attempts at something Rai-ish.

There are many old genealogies connecting the lineage to Rai or Bizen, but in reality its provincial Yamato offshoot.

Nothing clearly stating water, but often boshi area is more instructive.

Writing style, yasurime: Edo period Work: kind of Muromachi looking.

Looks like a nice Muromachi nakago with likely non-shoshin writing.

Its a bit tired, its not as impressive as the best Ryumon works, its his more Yamato than Bizen style, i.e. very Senjuin in style. The suriage is surprisingly recent and for some reason was accomponied by yasurime which was done without much dedication to the job. However without all these things we would be talking about what, TJ blade? So its all these arguments versus the price.

Boshi would reveal much. As is better picture of nioiguchi. Can be Mihara, but much more likely its Echizen Rai or someone related to it - from Rai Kunizane to Rai Kuniyasu.

Not the top but average Hizen or imitation. Yokoyama Bizen actually made good Hizen looking swords.

Possible, though with Uda of this type I would prefer to see a long kaeri.

Yamato style activities, Muromachi. The mei is ambitious but might paper with a note (Muromachi).

Yes its interesting and unusual. The hamon profile reminds one of late Muromachi. Strong nie activity like this in combination with nie hamon was seen in shinshinto and in late Muromachi, Momoyama included - some Bizen smiths at the time experimented with hard nie which forms ara nie, kinsuji, sunagashi etc..

Yamato=high shinogi. The school is known for thick kasane relative to others in Nambokucho period, mostly around 0.7cm but much larger thickness at shinogi, which if I remember correctly exceeds 1cm.

10mm at shinogi.

I would think not. Low shinogi the difference can be close to zero. In sue Bizen and a few others. Yamato is a definition of high shinogi. Typically has 0.6 (usually 0.7) -0.75cm kasane so 0.55 is very thin for it. It has shinogi kasane I think often in excess of 10mm but I don't have the books in front of me. So, no this blade does not sound like Yamato or high shinogi, but then its a very secondary information.

Its Edo period most likely, but having full nakago picture would help. Boshi contour is sort of visible but again could help.

I am not the person to talk about signatures, but things about this one I don't like. Still horimono is decently executed.