PNSSHOGUN

If memory serves me right these were purposefully cut at that angle to allow a slight tension to avoid falling out. It would also allow easier removal as the handles sit further away from the Fuchi. You will find this detail on many good quality Koshirae with Kozuka.

Is the blade made in Bizen Den?

Mon in this case refers to the Akihide trained group of smiths.

At the end of the day it's a real Japanese sword with decent looking Kai Gunto Koshirae, but if you look closer the cracks start to show. There are a number of dealers from Japan offering these sorts of pieces, the prices are certainly low but all sorts of skulduggery may have been applied to them. The polish is rather poor and typical for rush jobs that photograph well but obscure details The blade appears tired and has Ware (flaws) showing all over It may be a symptom of the polish but the Hamon appears rather weak Looking closer at the fittings, they may have been re-gilt

You're leaping to some pretty tenuous conclusions without demonstrating basic knowledge about Japanese swords.

Many of the swords listed have some pretty significant issues, with the prices reflecting that. The mounts aren't too bad on this but the blade is a mess.

Likely one of the many Tosho from the Akihide Mon.

Looked to be classic Bizen Yokoyama Sukesada.

Were these photos taken after a polish?

Rather unique, at a guess something thrown together for field service during the Meiji period.

Would welcome any further opinions on the third character, perhaps @Nobody or @SteveM could lend their expertise?

There is nothing here even remotely resembling a Yasukuni-To, clearly an oil tempered junker.

From the limited pictures of the Hamon it has a twinge of Ishido school, perhaps the cut off character could be the top of "是" for Korekazu?

Thank you Piers, perhaps the first obscured character is Fuji 藤?

My rudimentary skills are no match for this apart from the last character, could a kind soul please help with the first three Kanji?

Horimono can be added for a variety of reasons, some less noble than others. Poor quality Horimono are an easy fix for unsightly flaws or to simply "tart up" an otherwise unremarkable blade. When done correctly Horimono are absolutely beautiful and greatly enhance a sword.

Well that's certainly fine encouragement for other comments from members here. My apologies for attempting to clarify reliable information, rather than furthering inane tabloid level rumours about well respected martial artists and Tosho.

Some of the information presented in the book 'Samurai Swords for the Material Battle' seems spurious at best. Yoshichika was one of a few Tosho that made swords for Emperor Hirohito's Ascension Ceremony, there were worn (or given as gifts) to various nobles or dignitaries present at the ceremony. In regards to Nakayama Hakudo, where have you read about this supposed relationship with Tosho in exchange for free swords? He was a highly respected instructor and Tameshi-Shi who worked closely with numerous Tosho including Hayama Enshin, Watanabe Kanenaga, Kojima Kanemichi, and Kurihara Akihide, to improve their sharpness and strength for cutting. Boiling these relationships down to something so mundane is rather tactless.

For active rust on the Nakago best to stabilise with a light gun oil and GENTLY rub down with a cotton rag until no more red rust appears. Please do not attempt anything more to these, as you will quickly ruin historical value. Depending on where this museum is located there may be someone close by that can provide advice.

As you've found out it's all too easy to spend large sums on very little. Invest in some patience now and decide what really interests you.

Navy Kyu Gunto Koshirae with Mon on the screw type Mekugi, typically seen on Naval Dirks:

A curious example on a Fujiwara Kanenaga sword where a Mon has been engraved on the blade, authenticity unknown:

If you're after an original WWII sword then getting a return would be a good idea. There is a huge amount to learn about Japanese swords and original Gunto fittings, as you're likely well aware now.

See Horikawa Kunihiro school. https://www.sho-shin.com/sai19.htm

Swords by Kanemasa (Baron Murata Tsuneyoshi's Tosho name) can sometimes be found with stamped numbers from this period.